

###Thermoelectric properties of two-dimensional Dirac materials|Eddwi H. Hasdeo,Lukas P. A. Krisna,Ahmad R. T. Nugraha###

Thermoelectric properties of two-dimensional Dirac materials. We performed Boltzmann transport calculation to obtain the Seebeck
coefficient, electrical conductivity, electronic thermal conductivity, and
thermoelectric figure of merit (ZT) for Dirac systems. We found an enhancement
of ZT due to the gap opening. When the phonon thermal conductivity is small
enough, the optimum ZT in gapped Dirac system can be larger than 1, which is
preferable for thermoelectric applications.

###Efficiency of Energy Conversion in Thermoelectric Nanojunctions|Yu-Shen Liu,Yi-Ren Chen,Yu-Chang Chen###

Efficiency of Energy Conversion in Thermoelectric Nanojunctions. Using first-principles approaches, this study investigated the efficiency of
energy conversion in nanojunctions, described by the thermoelectric figure of
merit $ZT$. We obtained the qualitative and quantitative descriptions for the
dependence of $ZT$ on temperatures and lengths. A characteristic temperature:
$T_{0}= \sqrt{\beta/\gamma(l)}$ was observed. When $T\ll T_{0}$, $ZT\propto
T^{2}$. When $T\gg T_{0}$, $ZT$ tends to a saturation value. The dependence of
$ZT$ on the wire length for the metallic atomic chains is opposite to that for
the insulating molecules: for aluminum atomic (conducting) wires, the
saturation value of $ZT$ increases as the length increases; while for
alkanethiol (insulating) chains, the saturation value of $ZT$ decreases as the
length increases. $ZT$ can also be enhanced by choosing low-elasticity bridging
materials or creating poor thermal contacts in nanojunctions. The results of
this study may be of interest to research attempting to increase the efficiency
of energy conversion in nano thermoelectric devices.

###Low-Temperature Thermoelectric Performance and Optoelectronic Properties of Monolayer of WX2N4(X = Si, Ge)|Chayan Das,Dibyajyoti Saikia,Atanu Betal,Satyajit Sahu###

Low-Temperature Thermoelectric Performance and Optoelectronic Properties of Monolayer of WX2N4(X = Si, Ge). We investigated the thermoelectric properties of the 2D monolayer of WX2N4
using Density Functional Theory combined with Boltzmann Transport Equation. We
obtained an outstanding thermoelectric figure of merit of 0.91 at 400K for
p-type WGe2N4, whether it showed a ZT value of 0.56 for n-type at the same
temperature. On the other hand, the WSi2N4 showed significantly low ZT at room
temperature.

###Theory of ZT enhancement in nanocomposite materials|Paul M. Haney###

Theory of ZT enhancement in nanocomposite materials. The effect of interface scattering on the performance of disordered,
nanocomposite thermoelectric materials is studied theoretically using effective
medium theory and direct numerics. The interfacial electronic and phonon
scattering properties which lead to an enhancement of the thermoelectric figure
of merit $ZT$ are described. Generally, $ZT$ enhancement requires the
interfacial electrical conductance to be within a range of values, and the
thermal phonon conductance to be below a critical value. For the systems
considered, these requirements on interface scattering for $ZT$ enhancement are
expressed in terms of the bulk properties of the high-$ZT$ material, and the
ratio of the constituent bulk $Z$ values.

###Driving Perpendicular Heat Flow: Ambipolar Transverse Thermoelectrics for Microscale and Cryogenic Peltier Cooling|Chuanle Zhou,S. Birner,Yang Tang,K. Heinselman,M. Grayson###

Driving Perpendicular Heat Flow: Ambipolar Transverse Thermoelectrics for Microscale and Cryogenic Peltier Cooling. Whereas thermoelectric performance is normally limited by the figure of merit
ZT, transverse thermoelectrics can achieve arbitrarily large temperature
differences in a single leg even with inferior ZT by being geometrically
tapered. We introduce a band-engineered transverse thermoelectric with p-type
Seebeck in one direction and n-type orthogonal, resulting in off-diagonal terms
that drive heat flow transverse to electrical current. Such materials are
advantageous for microscale devices and cryogenic temperatures -- exactly the
regimes where standard longitudinal thermoelectrics fail. InAs/GaSb type II
superlattices are shown to have the appropriate band structure for use as a
transverse thermoelectric.

###Effect of Sb deficiency on the thermoelectric properties of Zn4Sb3|Anup V. Sanchela,Ajay D. Thakur,C. V. Tomy###

Effect of Sb deficiency on the thermoelectric properties of Zn4Sb3. We have investigated the effect of Sb-deficiency on the thermoelectric figure
of merit (zT) of Zn4Sb3 prepared by solid state reaction route. At high
temperatures, the Seebeck coefficient (S) and electrical conductivity
({\sigma}) increase with increase in Sb deficiency whereas the thermal
conductivity (\k{appa}) decreases giving rise to an increase in the overall zT
value. The observations suggest that creation of vacancies could be an
effective route in improving the thermoelectric properties of Zn4Sb3 system.
This coupled to nanostructuring strategy could lead to the ultimate maximum
value of zT in this system for high temperature thermoelectric applications.

###Metal halide thermoelectrics: prediction of high-performance CsCu2I3|Jong Woong Park,Young-Kwang Jung,Aron Walsh###

Metal halide thermoelectrics: prediction of high-performance CsCu2I3. Thermoelectric devices can directly convert waste heat into electricity,
which makes them an important clean energy technology. The underlying materials
performance can be evaluated by the dimensionless figure of merit ZT. Metal
halides are attractive candidates due to their chemical flexibility and ease of
processing; however, the maximum ZT realized (ZT = 0.15) falls far below the
level needed for commercialization (ZT > 1). Using a first-principles procedure
we assess the thermoelectric potential of copper halide CsCu2I3, which features
1D Cu-I connectivity. The n-type crystal is predicted to exhibit a maximum ZT
of 2.2 at 600 K along the b-axis. The strong phonon anharmonicity of this
system is shown by locally stable non-centrosymmetric Amm2 structures that are
averaged to form the observed centrosymmetric Cmcm space group. Our work
provides insights into the structure-property relations in metal halide
thermoelectrics and suggests a path forward to engineer higher-performance
heat-to-electricity conversion.

###Fluctuation-Dissipation in Thermoelectrics|Ngoc Anh Minh Tran,Aditya Savitha Dutt,Nithin Bharadwaj Pulumati,Heiko Reith,Anjun Hu,Alexandre Dumont,Kornelius Nielsch,André-Marie Tremblay,Gabi Schierning,Bertrand Reulet,Thomas Szkopek###

Fluctuation-Dissipation in Thermoelectrics. Thermoelectric materials exhibit correlated transport of charge and heat. The
Johnson-Nyquist noise formula $ 4 k_B T R $ for spectral density of voltage
fluctuations accounts for fluctuations associated solely with Ohmic
dissipation. Applying the fluctuation-dissipation theorem, we generalize the
Johnson-Nyquist formula for thermoelectrics, finding an enhanced voltage
fluctuation spectral density $4 k_B T R (1 + ZT)$ at frequencies below a
thermal cut-off frequency $f_T$, where $ZT$ is the dimensionless thermoelectric
material figure of merit. The origin of the enhancement in voltage noise is
thermoelectric coupling of temperature fluctuations. We use a wideband
($f_T\sim1$ kHz), integrated thermoelectric micro-device to experimentally
confirm our findings. Measuring the $ZT$ enhanced voltage noise, we
experimentally resolve temperature fluctuations with an amplitude of $0.8~\mu
\mathrm{K} \mathrm{Hz}^{-1/2}$ at a mean temperature of 295 K. We find that
thermoelectric devices can be used for thermometry with sufficient resolution
to measure the fundamental temperature fluctuations described by the
fluctuation-dissipation theorem.

###Enhanced thermoelectric properties in hybrid graphene-boron nitride nanoribbons|Kaike Yang,Yuanping Chen,Roberto D'Agosta,Yuee Xie,Jianxin Zhong,Angel Rubio###

Enhanced thermoelectric properties in hybrid graphene-boron nitride nanoribbons. The thermoelectric properties of hybrid graphene-boron nitride nanoribbons
(BCNNRs) are investigated using the non-equilibrium Green's function (NEGF)
approach. We find that the thermoelectric figure of merit (ZT) can be
remarkably enhanced by periodically embedding hexagonal BN (h-BN) into graphene
nanoribbons (GNRs). Compared to pristine GNRs, the ZT for armchair-edged BCNNRs
with width index 3p+2 is enhanced up to 10~20 times while the ZT of nanoribbons
with other widths is enhanced just by 1.5~3 times. As for zigzag-edge
nanoribbons, the ZT is enhanced up to 2~3 times. This improvement comes from
the combined increase in the Seebeck coefficient and the reduction in the
thermal conductivity outweighing the decrease in the electrical conductance. In
addition, the effect of component ratio of h-BN on the thermoelectric transport
properties is discussed. These results qualify BCNNRs as a promising candidate
for building outstanding thermoelectric devices.

###Enhanced thermoelectric performance in TiNiSn-based half-Heuslers|R. A. Downie,D. A. MacLaren,R. I. Smith,J. W. G. Bos###

Enhanced thermoelectric performance in TiNiSn-based half-Heuslers. Thermoelectric figures of merit, ZT > 0.5, have been obtained in arc-melted
TiNiSn-based ingots. This promising conversion efficiency is due to a low
lattice thermal conductivity, which is attributed to excess nickel in the
half-Heusler structure.

###Optimal band gap for improved thermoelectric performance of two-dimensional Dirac materials|Eddwi H. Hasdeo,Lukas P. A. Krisna,Muhammad Y. Hanna,Bobby E. Gunara,Nguyen T. Hung,Ahmad. R. T. Nugraha###

Optimal band gap for improved thermoelectric performance of two-dimensional Dirac materials. Thermoelectric properties of two-dimensional (2D) Dirac materials are
calculated within linearized Boltzmann transport theory and relaxation time
approximation. We find that the gapless 2D Dirac material exhibits poorer
thermoelectric performance than the gapped one. Furthermore, there exists an
optimal band gap for maximizing the figure of merit (ZT) in the gapped 2D Dirac
material. The optimal band gap ranges from 6kBT to 18kBT, where kB is the
Boltzmann constant and T is the operating temperature in kelvin. This result,
which is similar to that for bulk semiconductors, indicates the importance of
having narrow gaps to achieve the best thermoelectrics in 2D systems. Larger
maximum ZTs can also be obtained by suppressing the lattice thermal
conductivity. In the most ideal case where the lattice thermal conductivity is
zero (leaving the electron thermal conductivity alone), the maximum ZT in the
gapped 2D Dirac material is many times ZT of commercial thermoelectric
materials.

###Porosity-mediated High-performance Thermoelectric Materials|Xiaoxi Chen,Siyi Chang,Jin Chen,Pengfei Nan,Hao Wang,Shan Li,Xiyang Li,Xu Chen,Qiulin Liu,Xiaoshan Zhu,Binghui Ge,Wei Cai,Jiehe Sui,Shuqi Zheng,Fangwei Wang,Xiaolong Chen,Huaizhou Zhao###

Porosity-mediated High-performance Thermoelectric Materials. Whether porosity can effectively improve thermoelectric performance is still
an open question. Herein we report that thermoelectric performance can be
significantly enhanced by creating porosity in n-type
Mg3.225Mn0.025Sb1.5Bi0.49Te0.01, with a ZT of ~0.9 at 323 K and ~1.6 at 723 K,
making the average ZT much higher for better performance. The large improvement
at room temperature is significant considering that such a ZT value is
comparable to the best ZT at this temperature in n-type Bi2Te3. The enhancement
was mainly from the improved electrical mobility and multi-scale phonon
scattering, particularly from the well-dispersed bismuth nano-precipitates in
the porous structure. We further extend this approach to other thermoelectric
materials such as half-Heuslers Nb0.56V0.24Ti0.2FeSb and
Hf0.25Zr0.75NiSn0.99Sb0.01 and Bi0.5Sb1.5Te3 showing similar improvements,
further advancing thermoelectric materials for applications.

###Limitations of zT as a Figure of Merit for Nanostructured Thermoelectric Materials|Xufeng Wang,Mark Lundstrom###

Limitations of zT as a Figure of Merit for Nanostructured Thermoelectric Materials. A numerical study of thermoelectric nanocomposites is presented.
Thermoelectric properties as a function of average grain size or nanoparticle
density are examined by simulating the measurements as they would be done
experimentally. In accordance with previous theory and experimental results, we
find that the Seebeck coefficient, power factor and figure of merit, zT, can be
increased by nanostructuring when energy barriers exist around the grain
boundaries or embedded nanoparticles. When we simulate the performance of a
thermoelectric cooler with the same material, however, we find that the maximum
temperature difference is much less than expected from the given zT. The same
nanocomposite without electrically active grain boundaries has a lower measured
zT but a higher maximum cooling temperature. The physical reason for these
results is explained. The results illustrate the limitations of zT as a figure
of merit for nanocomposites with electrically active grain boundaries and
suggest that the ability to enhance the electrical performance of
thermoelectric materials by nanostructuring is limited.

###Enhanced Figure of Merit in Bismuth-Antimony Fine-Grained Alloys at Cryogenic Temperatures|Sheng Gao,John Gaskins,Xixiao Hu,Kathleen Tomko,Patrick Hopkins,S. Joseph Poon###

Enhanced Figure of Merit in Bismuth-Antimony Fine-Grained Alloys at Cryogenic Temperatures. Thermoelectric (TE) materials research plays a vital role in
heat-to-electrical energy conversion and refrigeration applications.
Bismuth-antimony (Bi-Sb) alloy is a promising material for thermoelectric
cooling. Herein, a high figure of merit, ZT, near 0.6 at cryogenic temperatures
(100-150K) has been achieved in melt-spun n-type Bi85Sb15 bulk samples
consisting of micron-size grains. The achieved ZT is nearly 50 percents higher
than polycrystalline averaged single crystal ZT of ~0.4, and it is also
significantly higher than ZT of less than ~0.3 measured below 150K in Bi-Te
alloys commonly used for cryogenic cooling applications. The improved
thermoelectric properties can be attributed to the fine-grained microstructure
achieved from rapid solidification, which not only significantly reduced the
thermal conductivity but also mitigated a segregation effect. A record low
thermal conductivity of ~1.5 W m-1 K-1 near 100 K was measured using the hot
disk method. The thermoelectric properties for this intriguing
semimetal-semiconductor alloy system were analyzed within a two-band effective
mass model. The study revealed a gradual narrowing of the band gap at
increasing temperature in Bi-Sb alloy for the first time.
Magneto-thermoelectric effects of this Bi-Sb alloy further improved the TE
properties, leading to ZT of about 0.7. The magneto-TE effect was further
demonstrated in a combined NdFeB/BiSb/NdFeB system. The compactness of the
BiSb-magnet system with high ZT enables the utilization of magneto-TE effect in
thermoelectric cooling applications.

###Enhanced Thermoelectric Performance and Anomalous Seebeck Effects in Topological Insulators|Yong Xu,Zhongxue Gan,Shou-Cheng Zhang###

Enhanced Thermoelectric Performance and Anomalous Seebeck Effects in Topological Insulators. Improving the thermoelectric figure of merit zT is one of the greatest
challenges in material science. Recent discovery of topological insulators
(TIs) offers new promise in this prospect. In this work, we demonstrate
theoretically that zT is strongly size dependent in TI, and the size parameter
can be tuned to enhance zT to be significantly greater than 1. Furthermore, we
show that the life time of the edge states in TI is strongly energy dependent,
leading to large and anomalous Seebeck effects with an opposite sign to the
Hall effect. These striking properties make TIs the promising material for
thermoelectrics science and technology.

###Thermoelectric properties of $n$-type SrTiO3|Jifeng Sun,David J. Singh###

Thermoelectric properties of $n$-type SrTiO3. We present an investigation of the thermoelectric properties of cubic
perovskite SrTiO3. The results are derived from a combination of calculated
transport functions obtained from Boltzmann transport theory in the constant
scattering time approximation based on the electronic structure and existing
experimental data for La-doped SrTiO3. The figure of merit ZT is modeled with
respect to carrier concentration and temperature. The model predicts a
relatively high $ZT$ at optimized doping, and suggests that the $ZT$ value can
reach 0.7 at T = 1400 K. Thus $ZT$ can be improved from the current
experimental values by carrier concentration optimization.

###Enhancement in the Figure of Merit of p-type BiSb alloys through multiple valence-band doping|Hyungyu Jin,Christopher M. Jaworski,Joseph P. Heremans###

Enhancement in the Figure of Merit of p-type BiSb alloys through multiple valence-band doping. N-type Bi100-xSbx alloys have the highest thermoelectric figure of merit (zT)
of all materials below 200K; here we investigate how filling multiple valence
band pockets at T and H-points of the Brillouin zone produces high zT in p-type
Sn-doped material. This approach, theoretically predicted to potentially give
zT>1 in Bi, was used successfully in PbTe. We report thermopower, electrical
and thermal conductivity (2 to 400K) of single crystals with 12<x<37 and
polycrystals (x=50-90), higher Sb concentrations than previous studies. We
obtain a 60% improvement in zT to 0.13.

###Gate voltage controlled thermoelectric figure of merit in three-dimensional topological insulator nanowires|Ning-Xuan Yang,Yan-Feng Zhou,Peng Lv,Qing-Feng Sun###

Gate voltage controlled thermoelectric figure of merit in three-dimensional topological insulator nanowires. The thermoelectric properties of the surface states in three-dimensional
topological insulator nanowires are studied. The Seebeck coefficients $S_c$ and
the dimensionless thermoelectrical figure of merit $ZT$ are obtained by using
the tight-binding Hamiltonian combining with the nonequilibrium Green's
function method. They are strongly dependent on the gate voltage and the
longitudinal and perpendicular magnetic fields. By changing the gate voltage or
magnetic fields, the values of $S_c$ and $ZT$ can be easily controlled. At the
zero magnetic fields and zero gate voltage, or at the large perpendicular
magnetic field and nonzero gate voltage, $ZT$ has the large value. Owing to the
electron-hole symmetry, $S_c$ is an odd function of the Fermi energy while $ZT$
is an even function regardless of the magnetic fields. $S_c$ and $ZT$ show
peaks when the quantized transmission coefficient jumps from one plateau to
another. The highest peak appears while the Fermi energy is near the Dirac
point. At the zero perpendicular magnetic field and zero gate voltage, the
height of $n$th peak of $S_C$ is $\frac{k_B}{e}\texttt{ln}2/(|n|+1/2)$ and
$\frac{k_B}{e}\texttt{ln}2/|n|$ for the longitudinal magnetic flux
$\phi_{\parallel} = 0 $ and $\pi$, respectively. Finally, we also study the
effect of disorder and find that $S_c$ and $ZT$ are robust against disorder. In
particular, the large value of $ZT$ can survive even if at the strong disorder.
These characteristics (that $ZT$ has the large value, is easily regulated, and
is robust against the disorder) are very beneficial for the application of the
thermoelectricity.

###Counterintuitive example on relation between ZT and thermoelectric efficiency|Byungki Ryu,Jaywan Chung,Eun-Ae Choi,Pawel Ziolkowski,Eckhard Müller,SuDong Park###

Counterintuitive example on relation between ZT and thermoelectric efficiency. The thermoelectric figure of merit ZT, which is defined using electrical
conductivity, Seebeck coefficient, thermal conductivity, and absolute
temperature T, has been widely used as a simple estimator of the conversion
efficiency of a thermoelectric heat engine. When material properties are
constant or slowly varying with T, a higher ZT ensures a higher maximum
conversion efficiency of thermoelectric materials. However, as material
properties can vary strongly with T, efficiency predictions based on ZT can be
inaccurate, especially for wide-temperature applications. Moreover, although ZT
values continue to increase, there has been no investigation of the
relationship between ZT and the efficiency in the higher ZT regime. In this
paper, we report a counterintuitive situation by comparing two materials:
although one material has a higher ZT value over the whole operational
temperature range, its maximum conversion efficiency is smaller than that of
the other. This indicates that, for material comparisons, the evaluation of
exact efficiencies as opposed to a simple comparison of the ZTs is necessary in
certain cases.

###Thermoelectricity in Nanowires: A Generic Model|Shadyar Farhangfar###

Thermoelectricity in Nanowires: A Generic Model. By employing a Boltzmann transport equation and using an energy and size
dependent relaxation time ($\tau$) approximation (RTA), we evaluate
self-consistently the thermoelectric figure-of-merit $ZT$ of a quantum wire
with rectangular cross-section. The inferred $ZT$ shows abrupt enhancement in
comparison to its counterparts in bulk systems. Still, the estimated $ZT$ for
the representative Bi$_2$Te$_3$ nanowires and its dependence on wire parameters
deviate considerably from those predicted by the existing RTA models with a
constant $\tau$. In addition, we address contribution of the higher energy
subbands to the transport phenomena, the effect of chemical potential tuning on
$ZT$, and correlation of $ZT$ with quantum size effects (QSEs). The obtained
results are of general validity for a wide class of systems and may prove
useful in the ongoing development of the modern thermoelectric applications.

###Prediction of high zT in thermoelectric silicon nanowires with axial germanium heterostructures|Matthew Shelley,Arash A. Mostofi###

Prediction of high zT in thermoelectric silicon nanowires with axial germanium heterostructures. We calculate the thermoelectric figure of merit,
zT=S^2GT/(\kappa_l+\kappa_e), for p-type Si nanowires with axial Ge
heterostructures using a combination of first-principles density-functional
theory, interatomic potentials, and Landauer-Buttiker transport theory. We
consider nanowires with up to 8400 atoms and twelve Ge axial heterostructures
along their length. We find that introducing heterostructures always reduces
S^2G, and that our calculated increases in zT are predominantly driven by
associated decreases in \kappa_l. Of the systems considered, <111> nanowires
with a regular distribution of Ge heterostructures have the highest
figure-of-merit: zT=3, an order of magnitude larger than the equivalent
pristine nanowire. Even in the presence of realistic structural disorder, in
the form of small variations in length of the heterostructures, zT remains
several times larger than that of the pristine case, suggesting that axial
heterostructuring is a promising route to high-zT thermoelectric nanowires.

###New record of high ZT found in hybrid transition-metal-dichalcogenides|Yulou Ouyang,Yuee Xie,Zhongwei Zhang,Qing Peng,Yuanping Chen###

New record of high ZT found in hybrid transition-metal-dichalcogenides. The search for thermoelectrics with higher figures of merit (ZT) will never
stop due to the demand of heat harvesting. Single layer transition metal
dichalcogenides (TMD), namely MX2 (where M is a transition metal and X is a
chalcogen) that have electronic band gaps are among the new materials that have
been the focus of such research. Here, we investigate the thermoelectric
transport properties of hybrid armchair-edged TMDs nanoribbons, by using the
nonequilibrium Green's function technique combined with the first principles
and molecular dynamics methods. We find a ZT as high as 7.4 in hybrid
MoS2/MoSe2 nanoribbons at 800K, creating a new record for ZT. Moreover, the
hybrid interfaces by substituting X atoms are more efficient than those by
substituting M atoms to tune the ZT. The origin of such a high ZT of hybrid
nanoribbons is the high density of the grain boundaries: the hybrid interfaces
decrease thermal conductance drastically without a large penalty to electronic
conductance.

###Thermoelectric efficiency of single-molecule junctions with long molecular linkers|Natalya A. Zimbovskaya###

Thermoelectric efficiency of single-molecule junctions with long molecular linkers. We report results of theoretical studies of thermoelectric efficiency of
single-molecule junctions with long molecular linkers. The linker is simulated
by a chain of identical sites described using a tight-binding model. It is
shown that thermoelectric figure of merit ZT strongly depends on the bridge
length, being controlled by the lineshape of electron transmission function
within the tunnel energy range corresponding to HOMO/LUMO transport channel.
Using the adopted model we demonstrate that ZT may significantly increase as
the linker lengthens, and that gateway states on the bridge (if any) may
noticeably affect the length-dependent ZT. Temperature dependences of ZT for
various bridge lengths are analyzed. It is shown that broad minima emerge in ZT
versus temperature curves whose positions are controlled by the bridge lengths.

###Nanograined half-Heusler semiconductors as advanced thermoelectrics: an ab-initio high-throughput statistical study|Jesús Carrete,Natalio Mingo,Shidong Wang,Stefano Curtarolo###

Nanograined half-Heusler semiconductors as advanced thermoelectrics: an ab-initio high-throughput statistical study. Nanostructuring has spurred a revival in the field of direct thermoelectric
energy conversion. Nanograined materials can now be synthesized with higher
figures of merit (ZT) than the bulk counterparts. This leads to increased
conversion efficiencies. Despite considerable effort in optimizing the known
and discovering the unknown, technology still relies upon a few limited
solutions. Here we perform ab-initio modeling of ZT for 75 nanograined
compounds obtained by filtering down the 79,057 half-Heusler entries available
in the AFLOWLIB.org repository according to electronic and thermodynamic
criteria. For many of the compounds the $ZT$s are markedly above those
attainable with nanograined IV and III-V semiconductors. About 15% of them may
even outperform ZT~2 at high temperatures. Our analysis elucidates the origin
of the advantageous thermoelectric properties found within this broad material
class. We use machine learning techniques to unveil simple rules determining if
a nanograined half-Heusler compound is likely to be a good thermoelectric given
its chemical composition.

###Inhomogeneous Thermal Conductivity Enhances Thermoelectric Cooling|Tingyu Lu,Jun Zhou,Nianbei Li,Ronggui Yang,Baowen Li###

Inhomogeneous Thermal Conductivity Enhances Thermoelectric Cooling. We theoretically investigate the enhancement of thermoelectric cooling
performance in thermoelectric devices made of materials with inhomogeneous
thermal conductivity, beyond the usual practice of enhancing thermoelectric
figure of merit ZT. The dissipation of Joule heat in such thermoelectric
devices is asymmetric which can give rise to better thermoelectric cooling
performance. Although the thermoelectric figure of merit and the
coefficient-of-performance are only slightly enhanced, both the maximum cooling
power and the maximum cooling temperature difference can be enhanced
significantly. This finding can be used to increase the heat absorption at the
cold end. The asymmetric dissipation of Joule heat also leads to thermal
rectification.

###Enhanced thermoelectric properties of coaxial Bi2Te3/Sb2Te3 nanostructures studied by theoretical modeling|Qilin Gu###

Enhanced thermoelectric properties of coaxial Bi2Te3/Sb2Te3 nanostructures studied by theoretical modeling. Critical thermoelectric parameters including Seebeck coefficient, electrical
conductivity, thermal conductivity and figure of merit ZT of one-dimensional
coaxial Bi2Te3/Sb2Te3 nanocomposite were modeled by following the single
carrier pocket and sharp interface assumptions. A calculation scheme based on
Landauer approach, instead of commonly used Boltzmann transport equation (BTE)
with relaxation time approximation, was adopted to numerically obtain the
transmission functions which can be used to evaluate thermoelectric properties.
Considerable enhancement of ZT was obtained through our modeling and numerical
calculation, and the corresponding dependence of thermoelectric performance on
structure parameters was studied. Finally, the efficiency at the maximum power
condition for this 1-D system was also investigated.

###Homogenous $In_{x}Ga_{1-x}N$ alloys on ZnO substrates: A new approach for high performance thermoelectric materials|Yining Feng,Evan Witkoske,Bahadir Kucukgok,Yee Rui Koh,Ali Shakouri,Ian T. Ferguson,Na Lu###

Homogenous $In_{x}Ga_{1-x}N$ alloys on ZnO substrates: A new approach for high performance thermoelectric materials. High performance thermoelectric materials for wide-range temperature
applications still remains a challenge. In this study, we have produced
high-quality homogeneous $In_{0.32}Ga_{0.68}N$ on ZnO substrates, with no phase
separation at high Indium content, using metal organic chemical vapor
deposition for thermoelectric applications. A record high room temperature
figure of merit zT is obtained of 0.86, which is five times larger than that of
SiGe, the current state of the art high temperature thermoelectric material.
These materials are shown to have a nearly perfect doping concentration to
maximize zT regardless of the scattering mechanism. This almost one order of
magnitude increase in zT is due to large electrical conductivities from oxygen
co-doping as well as low thermal conductivities from alloy scattering. The
maximum power factor reached was $77.98x10^{-4} W/mK^{2}$ at 300K for
$In_{0.32}Ga_{0.68}N$ alloys at a carrier concentration $~6.25x10^{20}
cm^{-3}$. This work indicates that $In_{x}Ga_{1-x}N$ alloys have great
potential for thermoelectric applications especially at a high temperature
range.

###Synergistic approach towards reproducible high zT in superionic thermoelectric Ag2Te|Navita Jakhar,Nita Bisht,Ankita Katre,Surjeet Singh###

Synergistic approach towards reproducible high zT in superionic thermoelectric Ag2Te. Recently, the superionic thermoelectrics, which typify the novel
`phonon-liquid electron-crystal' concept, have attracted enormous attention due
to their ultralow thermal conductivity and high figure-of-merit (zT). However,
their high zT is generally obtained deep inside the superionic phase, e.g.,
near 1000~K in the Cu$_2$X (X: chalcogen atom) family where the superionic
transition is close to 400~K. At such high temperatures, the liquid-like flow
of the metal ions under an electric field or a temperature gradient, both of
which are integral to the working of a thermoelectric device, results in device
degradation. To harness the full potential of the superionic thermoelectrics,
it is, therefore, necessary to reach high zT at low temperatures where the
metal-ion diffusion is not an issue. Here, we present a novel
all-room-temperature route to fabricate 100\% dense, nanostructured Ag$_2$Te
with highly reproducible thermoelectric properties and a high zT of 1.2 at
570~K, i.e., merely 150~K above its superionic transition. The samples show a
broad particle-size distribution ranging from a few nm to a few $\mu$m. This
hierarchical nanostructuring is shown to suppress the thermal conductivity of
Ag$_2$Te beyond the phonon-liquid electron-crystal limit to ultralow values,
leading to a remarkable enhancement of 87\% in the zT over that of the ingot
sample. These values supersede the zT of any Ag$_2$Te previously reported. Our
results are supported by first-principles density functional theory
calculations of the electronic and thermal properties.

###Thermoelectric degrees of freedom determining thermoelectric efficiency|Byungki Ryu,Jaywan Chung,SuDong Park###

Thermoelectric degrees of freedom determining thermoelectric efficiency. Thermal energy can be directly converted to electrical energy as a result of
thermoelectric effects. Because this conversion realises clean energy
technology, such as waste heat recovery and energy harvesting, substantial
efforts have been made to search for thermoelectric materials. Under the belief
that the material figure of merit $zT$ represents the energy conversion
efficiencies of thermoelectric devices, various high peak-$zT$ materials have
been explored for half a century. However, thermoelectric properties vary
greatly with temperature $T$, so the single value $zT$ does not represent
device efficiency accurately. Here we show that the efficiency of
thermoelectric conversion is completely determined by \emph{three} parameters
$Z_{\mathrm{gen}}$, $\tau$, and $\beta$, which we call the \emph{thermoelectric
degrees of freedom}. The $Z_{\mathrm{gen}}$, which is an average of material
properties, is a generalisation of the traditional figure of merit. The $\tau$
and $\beta$, which reflect the gradients of the material properties, are
proportional to escaped heat caused by the Thomson effect and asymmetric Joule
heat, respectively. Our finding proposes new directions for achieving high
thermoelectric efficiency; increasing one of the thermoelectric degrees of
freedom results in higher efficiency. For example, thermoelectric efficiency
can be enhanced up to 176\% by tuning the thermoelectric degrees of freedom in
segmented legs, compared to the best efficiency of single-material legs.

###Simultaneously optimizing the interdependent thermoelectric parameters in Ce(Ni$_{1-x}$Cu$_x$)$_2$Al$_3$|Peijie Sun,Tusyoshi Ikeno,Toshio Mizushima,Yosikazu Isikawa###

Simultaneously optimizing the interdependent thermoelectric parameters in Ce(Ni$_{1-x}$Cu$_x$)$_2$Al$_3$. Substitution of Cu for Ni in the Kondo lattice system CeNi$_2$Al$_3$ results
in a simultaneous optimization of the three interdependent thermoelectric
parameters: thermoelectric power, electrical and thermal conductivities, where
the electronic change in conduction band induced by the extra electron of Cu is
shown to be crucial. The obtained thermoelectric figure of merit $zT$ amounts
to 0.125 at around 100 K, comparable to the best values known for Kondo
compounds. The realization of ideal thermoelectric optimization in
Ce(Ni$_{1-x}$Cu$_x$)$_2$Al$_3$ indicates that proper electronic tuning of Kondo
compounds is a promising approach to efficient thermoelectric materials for
cryogenic application.

###Thermoelectric Properties of Silicon Carbide Nanowires with Nitrogen Dopants and Vacancies|Zhuo Xu,Qing-Rong Zheng,Gang Su###

Thermoelectric Properties of Silicon Carbide Nanowires with Nitrogen Dopants and Vacancies. The thermoelectric properties of cubic zincblend silicon carbide nanowires
(SiCNWs) with nitrogen impurities and vacancies along [111] direction are
theoretically studied by means of atomistic simulations. It is found that the
thermoelectric figure of merit ZT of SiCNWs can be significantly enhanced by
doping N impurities together with making Si vacancies. Aiming at obtaining a
large ZT, we study possible energetically stable configurations, and disclose
that, when N dopants locate at the center, a small number of Si vacancies at
corners are most favored for n-type nanowires, while a large number of Si
vacancies spreading into the flat edge sites are most favored for p-type
nanowires. For the SiCNW with a diameter of 1.1 nm and a length of 4.6 nm, the
ZT value for the n-type is shown capable of reaching 1.78 at 900K. The
conditions to get higher ZT values for longer SiCNWs are also addressed.

###High intrinsic $ZT$ in InP$_3$ monolayer at room temperature|Shenghui Zhang,Xiaobin Niu,Yiqun Xie,Kui Gong,Hezhu Shao,Yibin Hu,Yin Wang###

High intrinsic $ZT$ in InP$_3$ monolayer at room temperature. Two-dimensional thermoelectric materials with a figure of merit $ZT$, which
is greater than 2.0 at room temperature, would be highly desirable in energy
conversion since the efficiency is competitive to conventional energy
conversion techniques. Here, we propose that the indium triphosphide (InP$_3$)
monolayer offers an extraordinary $ZT$ of 2.2 at 300 K by using quantum
calculations within the ballistic thermal transport region. A remarkably low
and isotropic phononic thermal conductivity is founded, which is due to flat
lattice vibration modes. This low thermal conductivity takes a major
responsibility to the impressively high $ZT$. Moreover, a large $ZT$ that is
greater than 1.5 can be maintained, even if a 1% mechanic extension is applied
on the lattice. These results suggest that the InP$_3$ monolayer is a promising
candidate for low dimensional thermoelectric applications.

###Large thermoelectric figure of merit in graphene layered devices at low temperature|Daniel Olaya,Mikel Hurtado-Morales,Daniel Gomez,Octavio Alejandro Castaneda-Uribe,Zhen-Yu. Juang,Yenny Hernandez###

Large thermoelectric figure of merit in graphene layered devices at low temperature. Nanostructured materials have emerged as an alternative to enhance the figure
of merit (ZT) of thermoelectric (TE) devices. Graphene exhibits a high
electrical conductivity (in-plane) that is necessary for a high ZT; however,
this effect is countered by its impressive thermal conductivity. In this work
TE layered devices composed of electrochemically exfoliated graphene (EEG) and
a phonon blocking material such as poly (3,4-ethylenedioxythiophene)polystyrene
sulfonate (PEDOT:PSS), polyaniline (PANI) and gold nanoparticles (AuNPs) at the
interface were prepared. The figure of merit, ZT, of each device was measured
in the cross-plane direction using the Transient Harman Method (THM) and
complemented with AFM-based measurements. The results show remarkable high ZT
values (0.81 < ZT < 2.45) that are directly related with the topography,
surface potential, capacitance gradient and resistance of the devices at the
nanoscale.

###Building the Principle of Thermoelectric ZT Enhancement|Shuang Tang,Mildred Dresselhaus###

Building the Principle of Thermoelectric ZT Enhancement. Thermoelectrics involves both the generation of electrical power from heat
flow, and the efficient refrigeration using electricity, which has been
intensively focused on for two decades. The performance of thermoelectric power
generation and refrigeration has been improved by many proposals since 1993,
which is, however, the currently not yet in an industrially competitive level.
The development of the entire filed has come to a puzzling point, where the
physical reason of why this energy conversion efficiency enhancing problem is
barely soluble, and the physical answer of how it should be re-formed to be
soluble are the most urgent questions in this field. We here answer these
questions and provide the guidance on how to design the of thermoelectric
figure of merit enhancement in various systems using different approaches for
future researches. We start from criticizing the current way of expression the
thermoelectric figure of merit ZT and then introduce the new concept of
pseudo-ZT, which can finally make it clear what variables really decides ZT,
and clarify the historical misunderstanding of many thermoelectric performance
enhancing proposals. Then we explore the most important question on what
variables really enhance ZT, and give the contemporary researchers a new
guidance on what is the correct direction of thermoelectric performance
improving for various systems.

###Tuning thermoelectric power factor by crystal-field and spin-orbit couplings in Kondo lattice materials|Seungmin Hong,Pouyan Ghaemi,Joel E. Moore,Philip W. Phillips###

Tuning thermoelectric power factor by crystal-field and spin-orbit couplings in Kondo lattice materials. We study thermoelectric transport at low temperatures in correlated Kondo
insulators, motivated by the recent observation of a high thermoelectric figure
of merit(ZT) in $FeSb_2$ at $T \sim 10 K$. Even at room temperature,
correlations have the potential to lead to high ZT, as in $YbAl_3$, one of the
most widely used thermoelectric metals. At low temperature correlation effects
are especially worthy of study because fixed band structures are unlikely to
give rise to the very small energy gaps $E_g \sim 5 kT$ necessary for a weakly
correlated material to function efficiently at low temperature. We explore the
possibility of improving the thermoelectric properties of correlated Kondo
insulators through tuning of crystal field and spin-orbit coupling and present
a framework to design more efficient low-temperature thermoelectrics based on
our results.

###MoS2 Nanoribbons Thermoelectric Generators|Abbas Arab,Qiliang Li###

MoS2 Nanoribbons Thermoelectric Generators. In this work, we have designed and simulated new thermoelectric generator
based on monolayer and few-layer MoS2 nanoribbons. The proposed thermoelectric
generator is composed of thermocouples made of both n-type and p-type MoS2
nanoribbon legs. Density Functional Tight-Binding Non-Equilibrium Green's
Function (DFTB-NEGF) method has been used to calculate the transmission
spectrum of MoS2 armchair and zigzag nanoribbons. Phonon transmission spectrum
are calculated based on parameterization of Stillinger-Weber potential.
Thermoelectric figure of merit, ZT, is calculated using these electronic and
phonon transmission spectrum. Monolayer and bilayer MoS2 armchair nanoribbons
are found to have the highest ZT value for p-type and n-type legs, repectively.
Moreover, we have compared the thermoelectric current of doped monolayer MoS2
armchair nanoribbons and SZi thin films. Results indicate that thermoelectric
current of MoS2 monolayer nanoribbons is several orders of magnitude higher
than that of Si thin films.

###Magnetic-field enhanced high-thermoelectric performance in topological Dirac semimetal Cd$_3$As$_2$ crystal|H. H. Wang,X. G. Luo,W. W. Chen,N. Z. Wang,B. Lei,F. B. Meng,C. Shang,L. K. Ma,T. Wu,X. Dai,Z. F. Wang,X. H. Chen###

Magnetic-field enhanced high-thermoelectric performance in topological Dirac semimetal Cd$_3$As$_2$ crystal. Thermoelectric materials can be used to convert heat to electric power
through the Seebeck effect. We study magneto-thermoelectric figure of merit
(ZT) in three-dimensional Dirac semimetal Cd$_3$As$_2$ crystal. It is found
that enhancement of power factor and reduction of thermal conductivity can be
realized at the same time through magnetic field although magnetoresistivity is
greatly increased. ZT can be highly enhanced from 0.17 to 1.1 by more than six
times around 350 K under a perpendicular magnetic field of 7 Tesla. The huge
enhancement of ZT by magnetic field arises from the linear Dirac band with
large Fermi velocity and the large electric thermal conductivity in
Cd$_3$As$_2$. Our work paves a new way to greatly enhance the thermoelectric
performance in the quantum topological materials.

###Thermoelectric efficiency of topological insulators in a magnetic field|O. A. Tretiakov,Ar. Abanov,Jairo Sinova###

Thermoelectric efficiency of topological insulators in a magnetic field. We study the thermoelectric properties of three-dimensional topological
insulators in magnetic fields with many holes (or pores) in the bulk. We find
that at high density of these holes in the transport direction the
thermoelectric figure of merit, ZT, can be large due to the contribution of the
topologically protected conducting surfaces and the suppressed phonon thermal
conductivity. By applying an external magnetic field a subgap can be induced in
the surface states spectrum. We show that the thermoelectric efficiency can be
controlled by this tunable subgap leading to the values of ZT much greater than
1. Such high values of ZT for reasonable system parameters and its tunability
by magnetic field make this system a strong candidate for applications in heat
management of nanodevices, especially at low temperatures.

###Thermoelectric properties of Bi2Te3 atomic quintuple thin films|Ferdows Zahid,Roger Lake###

Thermoelectric properties of Bi2Te3 atomic quintuple thin films. Motivated by recent experimental realizations of quintuple atomic layer films
of Bi2Te3,the thermoelectric figure of merit, ZT, of the quintuple layer is
calculated and found to increase by a factor of 10 (ZT = 7.2) compared to that
of the bulk at room temperature. The large enhancement in ZT results from the
change in the distribution of the valence band density of modes brought about
by the quantum confinement in the thin film. The theoretical model uses ab
initio electronic structure calculations (VASP) with full quantum-mechanical
structure relaxation combined with a Landauer formalism for the linear-response
transport coefficients.

###Tuning of the Thermoelectric Figure of Merit of CH$_3$NH$_3$MI$_3$ (M=Pb,Sn) Photovoltaic Perovskites|Xavier Mettan,Riccardo Pisoni,Péter Matus,Andrea Pisoni,Jaćim Jaćimović,Bálint Náfrádi,Massimo Spina,Davor Pavuna,László Forró,Endre Horváth###

Tuning of the Thermoelectric Figure of Merit of CH$_3$NH$_3$MI$_3$ (M=Pb,Sn) Photovoltaic Perovskites. The hybrid halide perovskites, the very performant compounds in photovoltaic
applications, possess large Seebeck coefficient and low thermal conductivity
making them potentially interesting high figure of merit ($ZT$) materials. For
this purpose one needs to tune the electrical conductivity of these
semiconductors to higher values. We have studied the CH$_3$NH$_3$MI$_3$
(M=Pb,Sn) samples in pristine form showing very low $ZT$ values for both
materials; however, photoinduced doping (in M=Pb) and chemical doping (in M=Sn)
indicate that, by further doping optimization, $ZT$ can be enhanced toward
unity and reach the performance level of the presently most efficient
thermoelectric materials.

###Uncovering High Thermoelectric Figure of Merit in (Hf,Zr)NiSn Half-Heusler Alloys|L. Chen,S. Gao,X. Zeng,A. M. Dehkordi,T. M Tritt,S. J. Poon###

Uncovering High Thermoelectric Figure of Merit in (Hf,Zr)NiSn Half-Heusler Alloys. Half-Heusler alloys (MgAgSb structure) are promising thermoelectric
materials. RNiSn half-Heusler phases (R=Hf, Zr, Ti) are the most studied in
view of their thermal stability. The highest dimensionless figure of merit (ZT)
obtained is ~1 in the temperature range ~450-900oC, primarily achieved in
nanostructured alloys. Through proper annealing, ZT~1.2 has been obtained in a
previous ZT~1 n-type (Hf,Zr)NiSn phase without the nanostructure. There is an
appreciable increase in the power factor, decrease in charge carrier density,
and increase in carrier mobility. The findings are attributed to the
improvement of structural order. Present approach may be applied to optimize
the functional properties of Heusler-type alloys.

###Thermoelectric enhanced ZT regime calculated by Fermi integral method|Hirofumi Kakemoto###

Thermoelectric enhanced ZT regime calculated by Fermi integral method. We report about detailed dimensionless figure of merit ($ZT$) calculated by
using Fermi integral method (compared with Bi$_2$Te$_3$, CoSb$_3$, and
SrTiO$_3$) for thermoelectric (TE) materials' design and its module
application. Particularly, TE properties: electrical conductivity (small
polaron: $\sigma$($m$*)), Seebeck coefficient ($S$), thermal conductivity
($\kappa_e$), and $ZT$ were calculated by using reduced energy
($\zeta$=$E$/$k_B T$), as the functions of $T$, and effective mass ($m$*/$m$).
Enhancement of $ZT$ was investigated by contour plots of $T$, and $m$*/$m$
versus $\zeta$.

###Best Thermoelectric Efficiency of Ever-Explored Materials|Byungki Ryu,Jaywan Chung,Masaya Kumagai,Tomoya Mato,Yuki Ando,Sakiko Gunji,Atsumi Tanaka,Dewi Yana,Masayuki Fujimoto,Yoji Imai,Yukari Katsura,SuDong Park###

Best Thermoelectric Efficiency of Ever-Explored Materials. A thermoelectric device is a heat engine that directly converts heat into
electricity. Many materials with a high figure of merit ZT have been discovered
in anticipation of a high thermoelectric efficiency. However, there has been a
lack of investigations on efficiency-based material evaluation, and little is
known about the achievable limit of thermoelectric efficiency. Here, we report
the highest thermoelectric efficiency using 12,645 published materials. The
97,841,810 thermoelectric efficiencies are calculated using 808,610 device
configurations under various heat-source temperatures (T_h) when the cold-side
temperature is 300 K, solving one-dimensional thermoelectric integral equations
with temperature-dependent thermoelectric properties. For infinite-cascade
devices, a thermoelectric efficiency larger than 33% (~1/3) is achievable when
T_h exceeds 1400 K. For single-stage devices, the best efficiency of 17.1%
(~1/6) is possible when T_h is 860 K. Leg segmentation can overcome this limit,
delivering a very high efficiency of 24% (~1/4) when T_h is 1100 K.

###Cross-plane enhanced thermoelectricity and phonon suppression in graphene/MoS2 van der Waals heterostructures|Hatef Sadeghi,Sara Sangtarash,Colin J. Lambert###

Cross-plane enhanced thermoelectricity and phonon suppression in graphene/MoS2 van der Waals heterostructures. The thermoelectric figures of merit of pristine two-dimensional materials are
predicted to be significantly less than unity, making them uncompetitive as
thermoelectric materials. Here we elucidate a new strategy that overcomes this
limitation by creating multi-layer nanoribbons of two different materials and
allowing thermal and electrical currents to flow perpendicular to their planes.
To demonstrate this enhancement of thermoelectric efficiency ZT, we analyse the
thermoelectric performance of monolayer molybdenum disulphide (MoS2) sandwiched
between two graphene monolayers and demonstrate that the cross-plane (CP) ZT is
significantly enhanced compared with the pristine parent materials. For the
parent monolayer of MoS2, we find that ZT can be as high as approximately 0.3,
whereas monolayer graphene has a negligibly small ZT. In contrast for the
graphene/MoS2/graphene heterostructure, we find that the CP ZT can be as large
as 2.8. One contribution to this enhancement is a reduction of the thermal
conductance of the van der Waals heterostructure compared with the parent
materials, caused by a combination of boundary scattering at the MoS2/graphene
interface which suppresses the phonons transmission and the lower Debye
frequency of monolayer MoS2, which filters phonons from the monolayer graphene.
A second contribution is an increase in the electrical conductance and Seebeck
coefficient associated with molybdenum atoms at the edges of the nanoribbons.

###Substantial enhancement in thermoelectric figure-of-merit of half Heusler ZrNiPb alloys|Amardeep Sagar,Aman Bhardwaj,Andrei Novitskii,Vladimir Khovaylo,Satyabrata Patnaik###

Substantial enhancement in thermoelectric figure-of-merit of half Heusler ZrNiPb alloys. Ternary half Heusler alloys are under intense investigations recently towards
achieving high thermoelectric figure-of-merit (ZT). Of particular interest is
the ZrNiPb based half Heusler (HH) alloy where an optimal value of ZT = 0.7 at
773 K has been achieved by co-doping Sn and Bi at Pb site. In this work, we
identify an excellent ZT of 1.3 in ZrNi1+xPb0.38Sn0.6Bi0.02 (x= 0.03, at 773 K)
composite alloy. This is achieved by synergistic modulation of electronic as
well as thermal properties via introduction of minor phase of full Heusler (FH)
in the HH matrix through compositional tuning approach. These Ni-rich
ZrNi1+xPb0.38Sn0.6Bi0.02 alloys were synthesized via Arc melting followed by
consolidation via Spark Plasma Sintering (SPS). These alloys were characterized
by XRD and SEM that shows formation of nanocomposites comprising of HH matrix
phase and FH secondary minor phases. Enhancement in ZT is mainly attributed to
a synchronized increase in power factor and about 25% decrease in its thermal
conductivity. The thermoelectric compatibility factor (S) is also calculated
for all samples. The theoretically calculated thermoelectric device efficiency
of best performing sample ZrNi1.03Pb0.38Sn0.6Bi0.02 is estimated to be 13.6%.
Our results imply that controlled fine tuning in HH compounds through
compositional tuning approach would lead to novel off-stoichiometric HH phases
with enhanced ZT value for efficient thermoelectric device fabrication.

###Thermoelectric properties of junctions between metal and models of strongly correlated semiconductors|Massimo Rontani,L. J. Sham###

Thermoelectric properties of junctions between metal and models of strongly correlated semiconductors. We study the thermopower of a junction between a metal and a strongly
correlated semiconductor. Both in the electronic ferroelectric regime and in
the Kondo insulator regime the thermoelectric figures of merit, ZT, of these
junctions are compared with that of the ordinary semiconductor. By inserting at
the interface one or two monolayers of atoms different from the bulk, with a
suitable choice of rare-earth elements very high values of ZT can be reached at
low temperatures. The potential of the junction as a thermoelectric device is
discussed.

###Size dependent thermoelectric properties of silicon nanowires|Lihong Shi,Donglai Yao,Gang Zhang,Baowen Li###

Size dependent thermoelectric properties of silicon nanowires. By using first-principles tight-binding electronic structure calculation and
Boltzmann transport equation, we investigate the size dependence of
thermoelectric properties of silicon nanowires (SiNWs). With cross section area
increasing, the electrical conductivity increases slowly, while the Seebeck
coefficient reduces remarkably. This leads to a quick reduction of cooling
power factor with diameter. Moreover, the figure of merit also decreases with
transverse size. Our results demonstrate that in thermoelectric application, NW
with small diameter is preferred. We also predict that isotopic doping can
increase the value of ZT significantly. With 50% 29Si doping (28Si0.529Si0.5
NW), the ZT can be increased by 31%.

###Large thermoelectric figure of merit for 3D topological Anderson insulators via line dislocation engineering|Oleg A. Tretiakov,Ar. Abanov,Shuichi Murakami,Jairo Sinova###

Large thermoelectric figure of merit for 3D topological Anderson insulators via line dislocation engineering. We study the thermoelectric properties of three-dimensional topological
Anderson insulators with line dislocations. We show that at high densities of
dislocations the thermoelectric figure of merit ZT can be dominated by
one-dimensional topologically-protected conducting states channeled through the
lattice screw dislocations in the topological insulator materials with a
non-zero time-reversal-invariant momentum such as Bi_{1-x}Sb_x. When the
chemical potential does not exceed much the mobility edge the ZT at room
temperatures can reach large values, much higher than unity for reasonable
parameters, hence making this system a strong candidate for applications in
heat management of nano-devices.

###Holey topological thermoelectrics|Oleg A. Tretiakov,Ar. Abanov,Jairo Sinova###

Holey topological thermoelectrics. We study the thermoelectric properties of three-dimensional topological
insulators with many holes (or pores) in the bulk. We show that at high density
of these holes the thermoelectric figure of merit ZT can be large due to the
contribution of the conducting surfaces and the suppressed phonon thermal
conductivity. The maximum efficiency can be tuned by an induced gap in the
surface states dispersion through tunneling or external magnetic fields. The
large values of ZT, much higher than unity for reasonable parameters, make this
system a strong candidate for applications in heat management of nanodevices,
especially at low temperatures.

###Giant thermoelectric figure of merit in multivalley high-complexity-factor LaSO|Roberta Farris,Francesco Ricci,Giulio Casu,Diana Dahliah,Geoffroy Hautier,Gian-Marco Rignanese,Vincenzo Fiorentini###

Giant thermoelectric figure of merit in multivalley high-complexity-factor LaSO. We report a giant thermoelectric figure of merit $ZT$ (up to 6 at 1100 K) in
$n$-doped lanthanum oxysulphate LaSO. Thermoelectric coefficients are computed
from ab initio bands within Bloch-Boltzmann theory in an energy-, chemical
potential- and temperature-dependent relaxation time approximation. The lattice
thermal conductivity is estimated from a model employing the ab initio phonon
and Gr\"uneisen-parameter spectrum. The main source of the large $ZT$ is the
significant power factor which correlates with a large band complexity factor.
We also suggest a possible $n$-type dopant for the material based on ab initio
calculations.

###Improved Thermoelectric Cooling Based on the Thomson Effect|G. Jeffrey Snyder,Raghav Khanna,Eric S. Toberer,Nicholas A. Heinz,Wolfgang Seifert###

Improved Thermoelectric Cooling Based on the Thomson Effect. Traditional thermoelectric Peltier coolers exhibit a cooling limit which is
primarily determined by the figure of merit, zT. Rather than a fundamental
thermodynamic limit, this bound can be traced to the difficulty of maintaining
thermoelectric compatibility. Self-compatibility locally maximizes the cooler's
coefficient of performance for a given zT and can be achieved by adjusting the
relative ratio of the thermoelectric transport properties that make up zT. In
this study, we investigate the theoretical performance of thermoelectric
coolers that maintain self-compatibility across the device. We find such a
device behaves very differently from a Peltier cooler, and term self-compatible
coolers "Thomson coolers" when the Fourier heat divergence is dominated by the
Thomson, as opposed to the Joule, term. A Thomson cooler requires an
exponentially rising Seebeck coefficient with increasing temperature, while
traditional Peltier coolers, such as those used commercially, have
comparatively minimal change in Seebeck coefficient with temperature. When
reasonable material property bounds are placed on the thermoelectric leg, the
Thomson cooler is predicted to achieve approximately twice the maximum
temperature drop of a traditional Peltier cooler with equivalent figure of
merit (zT). We anticipate the development of Thomson coolers will ultimately
lead to solid state cooling to cryogenic temperatures.

###Effect of hydrostatic pressure and alloying on thermoelectric properties of van der Waals solid KMgSb: An \textit{ab-initio} study|Vikrant Chaudhary,Tulika Maitra,Tashi Nautiyal,Jeroen van den Brink,Hem C. Kandpal###

Effect of hydrostatic pressure and alloying on thermoelectric properties of van der Waals solid KMgSb: An \textit{ab-initio} study. Through a combined first-principles and Boltzmann transport theory, we
systematically investigate the thermal and electrical transport properties of
the unexplored ternary quasi two-dimensional KMgSb system of KMgX (X = P, As,
Sb, and Bi) family. Herein, the transport properties of KMgSb under the
application of hydrostatic pressure and alloy engineering are reported. At a
carrier concentration of $\sim8\times10^{19}~\mathrm{cm^{-3}}$, the figure of
merit zT ($\sim0.75$) for both the $n$-type and $p$-type of KMgSb closely
matched, making it an attractive option for engineering both legs of a
thermoelectric device using the same material. This is particularly desirable
for high-performance thermoelectric applications. Furthermore, the zT value
increases as pressure decreases, further enhancing its potential for use in
thermoelectric devices. In the case of substitutional doping (replacing 50 \%
Sb by Bi atom), we observed $\sim49~\%$ (in-plane) increase in the peak
thermoelectric figure of merit (zT). The maximum zT value obtained after alloy
engineering is $\sim1.45$ at 900~K temperature. Hydrostatic pressure is
observed to be a great tool to tune the lattice thermal conductivity
($\kappa_L$). We observed that the negative pressure-like effects could be
achieved by chemically doping bigger-size atoms, especially when $\kappa_L$ is
a property under investigation. Through our computational investigation, we
explain that hydrostatic pressure and alloy engineering may improve
thermoelectric performance dramatically.

###Thermoelectric propertie of Ge based electron crystal phonon glass materials|Dimitri Tainoff,André Barski,Eric Prestat,Daniel Bourgault,Emmanuell Hadji,Yanqing Liu,Pascale Bayle-Guillemaud,Olivier Bourgeois###

Thermoelectric propertie of Ge based electron crystal phonon glass materials. We report on the elaboration of germanium manganese nanostructured thin films
and the measurement of their thermoelectric properties. We investigate the
growth of Ge:Mn layers along with a thorough structural characterization of
this materials at the nanoscale. The room temperature thermoelectric properties
of these layers containing spherical inclusions are discussed regarding their
potential as a model of "electron crystal phonon glass material". We show that
the thermal conductivity can be decreased by a factor of 30, even if the
electronic properties can be conserved as in the bulk. The thermoelectric
performance ZT of such material is as high as 0.15 making them a promising
thermoelectric p-type material for Ge related application.

###Thermal transport in the Falicov-Kimball model on a Bethe lattice|A. V. Joura,D. O. Demchenko,J. K. Freericks###

Thermal transport in the Falicov-Kimball model on a Bethe lattice. We calculate thermal transport in the Falicov-Kimball model on an
infinite-coordination-number Bethe lattice. We perform numerical calculations
of the thermoelectric characteristics and concentrate on finding materials
parameters for which the electronic thermoelectric figure-of-merit ZT is large,
suggesting potential cooling and power generation applications. Surprisingly,
the Bethe lattice has significant qualitative and quantitative differences with
the previously studied hypercubic lattice. At low temperature it is unlikely
that these systems can be employed in thermoelectric devices due to the low
conductivities and due to a larger lattice contribution to the thermal
conductivity $\kappa_L$, but at high temperature, the thermoelectric parameters
appear more promising for devices due to a significant enhancement of ZT and a
smaller relative contribution by the lattice thermal conductivity.

###Thermoelectric transport properties of a T-shaped double quantum dot system in the Coulomb blockade regime|A. L. Monteros,G. S. Uppal,S. R. McMillan,M. Crisan,I. Tifrea###

Thermoelectric transport properties of a T-shaped double quantum dot system in the Coulomb blockade regime. We investigate the thermoelectric properties of a T-shaped double quantum dot
system described by a generalized Anderson Hamiltonian. The system's electrical
conduction (G) and the fundamental thermoelectric parameters such as the
Seebeck coefficient ($S$) and the thermal conductivity ($\kappa$), along with
the system's thermoelectric figure of merit (ZT) are numerically estimated
based on a Green's function formalism that includes contributions up to the
Hartree-Fock level. Our results account for finite onsite Coulomb interaction
terms in both component quantum dots and discuss various ways leading to an
enhanced thermoelectric figure of merit for the system. We demonstrate that the
presence of Fano resonances in the Coulomb blockade regime is responsible for a
strong violation of the Wiedemann-Franz law and a considerable enhancement of
the system's figure of merit ($ZT$).

###High thermoelectric performance of graphite nanofibers|Van-Truong Tran,Jérôme Saint-Martin,Philippe Dollfus,Sebastian Volz###

High thermoelectric performance of graphite nanofibers. Graphite nanofibers (GNFs) have been demonstrated to be a promising material
for hydrogen storage and heat management in electronic devices. Here, by means
of first-principles and transport simulations, we show that GNFs can also be an
excellent material for thermoelectric applications thanks to the interlayer
weak van der Waals interaction that induces low thermal conductance and a
step-like shape in the electronic transmission with mini-gaps, which are
necessary ingredients to achieve high thermoelectric performance. This study
unveils that the platelet form of GNFs in which graphite layers are
perpendicular to the fiber axis can exhibit outstanding thermoelectric
properties with a figure of merit ZT reaching 3.55 in a 0.5 nm diameter fiber
and 1.1 for a 1.1 nm diameter one. Interestingly, by introducing 14C isotope
doping, ZT can even be enhanced up to more than 5, and more than 8 if we
include the effect of finite phonon mean-free path, which demonstrates the
amazing thermoelectric potential of GNFs.

###Enhanced thermoelectric performance of phosphorene by strain-induced band convergence|H. Y. Lv,W. J. Lu,D. F. Shao,Y. P. Sun###

Enhanced thermoelectric performance of phosphorene by strain-induced band convergence. The newly emerging monolayer phosphorene was recently predicted to be a
promising thermoelectric material. In this work, we propose to further enhance
the thermoelectric performance of phosphorene by the strain-induced band
convergence. The effect of the uniaxial strain on the thermoelectric properties
of phosphorene was investigated by using the first-principles calculations
combined with the semi-classical Boltzmann theory. When the zigzag-direction
strain is applied, the Seebeck coefficient and electrical conductivity in
zigzag direction can be greatly enhanced simultaneously at the critical strain
of 5% where the band convergence is achieved. The largest ZT value of 1.65 at
300 K is then achieved conservatively estimated by using the bulk lattice
thermal conductivity. When the armchair-direction strain of 8% is applied, the
room-temperature ZT value can reach 2.12 in the armchair direction of
phosphorene. Our results indicate that strain induced band convergence could be
an effective method to enhance the thermoelectric performance of phosphorene.

###Thermoelectric coefficients and the figure of merit for large open quantum dots|Robert S. Whitney,Keiji Saito###

Thermoelectric coefficients and the figure of merit for large open quantum dots. We consider the thermoelectric response of chaotic or disordered quantum dots
in the limit of phase-coherent transport, statistically described by random
matrix theory. We calculate the full distribution of the thermoelectric
coefficients (Seebeck $S$ and Peltier $\Pi$), and the thermoelectric figure of
merit $ZT$, for large open dots at arbitrary temperature and external magnetic
field, when the number of modes in the left and right leads ($N_{\rm L}$ and
$N_{\rm R}$) are large. Our results show that the thermoelectric coefficients
and $ZT$ are maximal when the temperature is half the Thouless energy, and the
magnetic field is negligible. They remain small, even at their maximum, but
they exhibit a type of universality at all temperatures, in which they do not
depend on the asymmetry between the left and right leads $(N_{\rm L}-N_{\rm
R})$, even though they depend on $(N_{\rm L}+N_{\rm R})$.

###Intrinsically high thermoelectric figure of merit of half-Heusler ZrRuTe|Sonu Prasad Keshri,Amal Medhi###

Intrinsically high thermoelectric figure of merit of half-Heusler ZrRuTe. The electronic structure and thermoelectric properties of ZrRuTe-based
Half-Heusler compounds are studied using density functional theory (DFT) and
Boltzmann transport formalism. Based on rigorous computations of electron
relaxation time $\tau$ considering electron-phonon interactions and lattice
thermal conductivity $\kappa_l$ considering phonon-phonon interactions, we find
ZrRuTe to be an intrinsically good thermoelectric material. It has a high power
factor of $\sim 2\times 10^{-3}$ W/m-K$^{2}$ and low $\kappa_l\sim 10$ W/m-K at
800 K. The thermoelectric figure of merit $ZT \sim 0.13$ at 800 K is higher
than similar other compounds. We have also studied the properties of the
material as a function of doping and find the thermoelectric properties to be
substantially enhanced for $p$-doped ZrRuTe with the $ZT$ value raised to $\sim
0.2$ at this temperature. The electronic, thermodynamic, and transport
properties of the material are thoroughly studied and discussed

###Thermoelectric transport with electron-phonon coupling and electron-electron interaction in molecular junctions|Jie Ren,Jian-Xin Zhu,James E. Gubernatis,Chen Wang,Baowen Li###

Thermoelectric transport with electron-phonon coupling and electron-electron interaction in molecular junctions. Within the framework of nonequilibrium Green's functions, we investigate the
thermoelectric transport in a single molecular junction with electron-phonon
and electron-electron interactions. By transforming into a displaced phonon
basis, we are able to deal with these interactions non-perturbatively. Then, by
invoking the weak tunneling limit, we are able to calculate the
thermoelectricity. Results show that at low temperatures, resonances of the
thermoelectric figure of merit ZT occur around the sides of resonances of
electronic conductance but drops dramatically to zero at exactly these resonant
points. We find ZT can be enhanced by increasing electron-phonon coupling and
Coulomb repulsion, and an optimal enhancement is obtained when these two
interactions are competing. Our results indicate a great potential for
single-molecular-junctions as good thermoelectric devices over a wide range of
temperatures.

###A one dimensional hard-point gas as a thermoelectric engine|Jiao Wang,Giulio Casati,Tomaz Prosen,C. -H. Lai###

A one dimensional hard-point gas as a thermoelectric engine. We demonstrate the possibility to build a thermoelectric engine using a one
dimensional gas of molecules with unequal masses and hard-point interaction.
Most importantly, we show that the efficiency of this engine is determined by a
new parameter YT which is different from the well known figure of merit ZT.
Even though the efficiency of this particular model is low, our results shed
new light on the problem and open the possibility to build efficient
thermoelectric engines.

###Surface decorated silicon nanowires: a route to high-ZT thermoelectrics|Troels Markussen,Antti-Pekka Jauho,Mads Brandbyge###

Surface decorated silicon nanowires: a route to high-ZT thermoelectrics. Based on atomistic calculations of electron and phonon transport, we propose
to use surface decorated Silicon nanowires (SiNWs) for thermoelectric
applications. Two examples of surface decorations are studied to illustrate the
underlying deas: Nanotrees and alkyl functionalized SiNWs. For both systems we
find, (i) that the phonon conductance is significantly reduced compared to the
electronic conductance leading to high thermoelectric figure of merit, $ZT$,
and (ii) for ultra-thin wires surface decoration leads to significantly better
performance than surface disorder.

###Giant Thermoelectric Effect from Transmission Supernodes|J. P. Bergfield,M. Solis,C. A. Stafford###

Giant Thermoelectric Effect from Transmission Supernodes. We predict an enormous order-dependent quantum enhancement of thermoelectric
effects in the vicinity of a higher-order `supernode' in the transmission
spectrum of a nanoscale junction. Single-molecule junctions based on
3,3'-biphenyl and polyphenyl ether (PPE) are investigated in detail. The
nonequilibrium thermodynamic efficiency and power output of a thermoelectric
heat engine based on a 1,3-benzene junction are calculated using many-body
theory, and compared to the predictions of the figure-of-merit ZT.

###Huge thermoelectric effects in ferromagnet-superconductor junctions in the presence of a spin-splitting field|A. Ozaeta,P. Virtanen,F. S. Bergeret,T. T. Heikkilä###

Huge thermoelectric effects in ferromagnet-superconductor junctions in the presence of a spin-splitting field. We show that a huge thermoelectric effect can be observed by contacting a
superconductor whose density of states is spin-split by a Zeeman field with a
ferromagnet with a non-zero polarization. The resulting thermopower exceeds
$k_B/e$ by a large factor, and the thermoelectric figure of merit $ZT$ can far
exceed unity, leading to heat engine efficiencies close to the Carnot limit. We
also show that spin-polarized currents can be generated in the superconductor
by applying a temperature bias.

###Predicting the optimized thermoelectric performance of MgAgSb|C. Y. Sheng,H. J. Liu,D. D. Fan,L. Cheng,J. Zhang J. Wei,J. H. Liang,P. H. Jiang,J. Shi###

Predicting the optimized thermoelectric performance of MgAgSb. Using first-principles method and Boltzmann theory, we provide an accurate
prediction of the electronic band structure and thermoelectric transport
properties of alpha-MgAgSb. Our calculations demonstrate that only when an
appropriate exchange-correlation functional is chosen can we correctly
reproduce the semiconducting nature of this compound. By fine tuning the
carrier concentration, the thermoelectric performance of alpha-MgAgSb can be
significantly optimized, which exhibits a strong temperature dependence and
gives a maximum ZT value of 1.7 at 550 K.

###Anisotropic thermoelectric properties of EuCd$_{2}$As$_{2}$ : An Ab-initio study|Jyoti Krishna,Mukesh Sharma,T. Maitra###

Anisotropic thermoelectric properties of EuCd$_{2}$As$_{2}$ : An Ab-initio study. In search of better thermoelectric materials, we have systematically
investigated the thermoelectric properties of a 122 Zintl phase compound
EuCd$_{2}$As$_{2}$ using \textit{ab-initio} density functional theory and
semi-classical Boltzmann transport theory within constant relaxation time
approximation. Considering the ground state magnetic structure which is A-type
antiferromagnetic (A-AFM) and non-magnetic (NM) structure, we evaluated various
thermoelectric parameters such as Seebeck coefficient, electrical and thermal
conductivity, power factor and figure of merit (ZT) as function temperature as
well as chemical potential. Almost all thermoelectric parameters show
anisotropy between $xx$ and $zz$ directions which is stronger in case of A-AFM
than in NM. Both A-AFM and NM phase of the compound display better
thermoelectric performance when hole doped. We observed high Seebeck
coefficient and low electronic thermal conductivity in A-AFM phase along $zz$
direction. The remarkably high ZT of 1.79 at 500 K in A-AFM phase and ZT$\sim$1
in NM phase suggest that EuCd$_{2}$As$_{2}$ is a viable thermoelectric material
when p-doped.

###Multiterminal single-molecule--graphene-nanoribbon thermoelectric devices with gate-voltage tunable figure of merit ZT|Kamal K. Saha,Troels Markussen,Kristian S. Thygesen,Branislav K. Nikolic###

Multiterminal single-molecule--graphene-nanoribbon thermoelectric devices with gate-voltage tunable figure of merit ZT. We study thermoelectric devices where a single 18-annulene molecule is
connected to metallic zigzag graphene nanoribbons (ZGNR) via highly transparent
contacts that allow for injection of evanescent wave functions from ZGNRs into
the molecular ring. Their overlap generates a peak in the electronic
transmission, while ZGNRs additionally suppress hole-like contributions to the
thermopower. Thus optimized thermopower, together with suppression of phonon
transport through ZGNR-molecule-ZGNR structure, yield the thermoelectric figure
of merit ZT ~ 0.5 at room temperature and 0.5 < ZT < 2.5 below liquid nitrogen
temperature. Using the nonequilibrium Green function formalism combined with
density functional theory, recently extended to multiterminal devices, we show
how the transmission resonance can also be manipulated by the voltage applied
to a third ZGNR electrode, acting as the top gate covering molecular ring, to
tune the value of ZT.

###On the Best Bandstructure for Thermoelectric Performance|Changwook Jeong,Raseong Kim,Mark Lundstrom###

On the Best Bandstructure for Thermoelectric Performance. The conventional understanding that a bandstructure that produces a Dirac
delta function transport distribution (or transmission in the Landauer
framework) maximizes the thermoelectric figure of merit, ZT, is revisited.
Thermoelectric (TE) performance is evaluated using a simple tight binding (TB)
model for electron dispersion and three different scattering models: 1) a
constant scattering time, 2) a constant mean-free-path, and 3) a scattering
rate proportional to the density-of-states. We found that a Dirac
delta-function transmission never produces the maximum ZT. The best
bandstructure for maximizing ZT depends on the scattering physics. These
results demonstrate that the selection of bandstructure to maximize TE
performance is more complex than previously thought and that a high
density-of-states near the band edge does not necessarily improve TE
performance.

###First-principles study of the thermoelectric properties of strained graphene nanoribbons|Pei Shan Emmeline Yeo,Michael B. Sullivan,Kian Ping Loh,Chee Kwan Gan###

First-principles study of the thermoelectric properties of strained graphene nanoribbons. We study the transport properties, in particular, the thermoelectric figure
of merit ZT of armchair graphene nanoribbons, AGNR-N (for N=4-12, with widths
ranging from 3.7 to 13.6~\AA) through strain engineering, where N is the number
of carbon dimer lines across the AGNR width. We find that the tensile strain
applied to AGNR-$N$ changes the transport properties by modifying the
electronic structures and phonon dispersion relations. The tensile strain
increases the ZT value of the AGNR-$N$ families with N=3p and N=3p+2, where $p$
is an integer. Our analysis based on accurate density-functional theory
calculations suggests a possible route to increase the ZT values of AGNR-$N$
for potential thermoelectric applications.

###Optimizing thermoelectric performances of low-temperature SnSe compounds by electronic structure design|Aijun Hong,Lin Li,Haixia Zhu,Zhibo Yan,Junming Liu,Zhifeng Ren###

Optimizing thermoelectric performances of low-temperature SnSe compounds by electronic structure design. Recently SnSe compound was reported to have a peak thermoelectric figure-5
of-merit (ZT) of 2.62 at 923 K, but the ZT values at temperatures below 750 K
are relatively low. In this work, the electronic structures of SnSe are
calculated using the density functional theory, and the electro- and
thermo-transport properties upon varying chemical potential (or carrier
density) are evaluated by the semi-classic Boltzmann transport theory, showing
that the calculated ZT values along the a10 and c-axes below 675 K are in
agreement with reported values, but that along the b-axis can be as high as
2.57 by optimizing the carrier concentration to ~3.6*1019 cm-3. It is revealed
that a mixed ionic-covalent bonding and heavy-light band overlapping near the
valence band are the reasons for the higher thermoelectric performance

###A spincaloritronic battery|Xiao-Qin Yu,Zhen-Gang Zhu,Gang Su,A. -P. Jauho###

A spincaloritronic battery. The thermoelectric performance of a topological energy converter is analyzed.
The H-shaped device is based on a combination of transverse topological effects
involving the spin: the inverse spin Hall effect and the spin Nernst effect.
The device can convert a temperature drop in one arm into an electric power
output in the other arm. Analytical expressions for the output voltage, the
figure-of-merit (ZT) and energy converting efficiency are reported. We show
that the output voltage and the ZT can be tuned by the geometry of the device
and the physical properties of the material. Importantly, contrary to a
conventional thermoelectric device, here a low electric conductivity may in
fact enhance the ZT value, thereby opening a path to new strategies in
optimizing the figure-of-merit.

###High thermoelectric figure of merit of quantum dot array quantum wires|David M T Kuo###

High thermoelectric figure of merit of quantum dot array quantum wires. How to design silicon-based quantum wires with figure of merit ($ZT$) larger
than three is under hot pursuit due to the advantage of low cost and the
availability of matured fabrication technique. Quantum wires consisting of
finite three dimensional quantum dot (QD) arrays coupled to electrodes are
proposed to realize high efficient thermoelectric devices with optimized power
factors. The transmission coefficient of 3D QD arrays can exhibit 3D, 2D, 1D
and 0D topological distribution functions by tailoring the interdot coupling
strengths. Such topological effects on the thermoelectric properties are
revealed. The 1D topological distribution function shows the maximum power
factor and the best $ZT$ value. We have demonstrated that 3D silicon QD array
nanowires with diameters below $20~nm$ and length $250~nm$ show high potential
to achieve $ZT\ge 3$ near room temperature.

###Thermoelectric figure of merit of tau-type conductors of several donors|H. Yoshino,H. Aizawa,K. Kuroki,G. C. Anyfantis,G. C. Papavassiliou,K. Murata###

Thermoelectric figure of merit of tau-type conductors of several donors. Dimensionless thermoelectric figure of merit $ZT$ is investigated for
two-dimensional organic conductors $\tau-(EDO-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$,
$\tau$-(EDT-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ and
$\tau$-(P-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ ($y \le 0.875$), respectively. The
$ZT$ values were estimated by measuring electrical resistivity, thermopower and
thermal conductivity simultaneously. The largest $ZT$ is 2.7 $\times$ 10$^{-2}$
at 155 K for $\tau-(EDT-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$, 1.5 $\times$ 10$^{-2}$
at 180 K for $\tau-(EDO-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ and 5.4 $\times$
10$^{-3}$ at 78 K for $\tau-(P-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$, respectively.
Substitution of the donor molecules fixing the counter anion revealed
EDT-S,S-DMEDT-TTF is the best of the three donors to obtain larger $ZT$.

###Enhancement of the thermoelectric figure of merit in a quantum dot due to the Coulomb blockade effect|Jie Liu,Qing-feng Sun,X. C. Xie###

Enhancement of the thermoelectric figure of merit in a quantum dot due to the Coulomb blockade effect. We investigate the figure of merit of a quantum dot (QD) in the Coulomb
blockade regime. It is found that the figure of merit $ZT$ may be quite high if
only single energy level in the QD is considered. On the other hand, with two
or multi energy levels in the QD and without the Coulomb interaction, the $ZT$
is strongly suppressed by the bipolar effect due to small level spacing.
However, in the presence of the Coulomb interaction, the effective level
spacing is enlarged and the bipolar effect is weakened, resulting in $ZT$ to be
considerably high. Thus, it is more likely to find a high efficient
thermoelectric QDs with large Coulomb interaction. By using the parameters for
a typical QD, the $ZT$ can reach over 5.

###Maximizing the thermoelectric performance of topological insulator Bi2Te3 films in the few-quintuple layer regime|Jinghua Liang,Long Cheng,Jie Zhang,Huijun Liu,Zhenyu Zhang###

Maximizing the thermoelectric performance of topological insulator Bi2Te3 films in the few-quintuple layer regime. Using first-principles calculations and Boltzmann theory, we explore the
feasibility to maximize the thermoelectric figure of merit (ZT) of topological
insulator Bi2Te3 films in the few-quintuple layer regime. We discover that the
delicate competitions between the surface and bulk contributions, coupled with
the overall quantum size effects, lead to a novel and generic non-monotonous
dependence of ZT on the film thickness. In particular, when the system crosses
into the topologically non-trivial regime upon increasing the film thickness,
the much longer surface relaxation time associated with the robust nature of
the topological surface states results in a maximal ZT value, which can be
further optimized to ~2.0 under physically realistic conditions. We also reveal
the appealing potential of bridging the long-standing ZT asymmetry of p- and
n-type Bi2Te3 systems.

###Theory of thermoelectricity in Mg$_3$Sb$_2$ with an energy- and temperature-dependent relaxation time|Roberta Farris,Maria Barbara Maccioni,Alessio Filippetti,Vincenzo Fiorentini###

Theory of thermoelectricity in Mg$_3$Sb$_2$ with an energy- and temperature-dependent relaxation time. We study the electronic transport coefficients and the thermoelectric figure
of merit ZT in $n$-doped Mg$_3$Sb$_2$ based on density-functional electronic
structure and Bloch-Boltzmann transport theory with an energy- and
temperature-dependent relaxation time. Both the lattice and electronic thermal
conductivities affect the final ZT significantly, hence we include the lattice
thermal conductivity calculated ab initio. Where applicable, our results are in
good agreement with existing experiments, thanks to the treatment of lattice
thermal conductivity and the improved description of electronic scattering. ZT
increases monotonically in our T range (300 to 700 K), reaching a value of 1.6
at 700 K; it peaks as a function of doping at about 3$\times$10$^{19}$
cm$^{-3}$. At this doping, ZT$>$1 for T$>$500 K.

###The Limits of Thermoelectric Performance with a Bounded Transport Distribution|Jesse Maassen###

The Limits of Thermoelectric Performance with a Bounded Transport Distribution. With the goal of maximizing the thermoelectric (TE) figure of merit $ZT$,
Mahan and Sofo [Proc. Natl. Acad. Sci. U.S.A. 93, 7436 (1996)] found that the
optimal transport distribution (TD) is a delta function. Materials, however,
have TDs that appear to always be finite and non-diverging. Motivated by this
observation, this study focuses on deriving what is the optimal bounded TD,
which is determined to be a boxcar function for $ZT$ and a Heaviside function
for power factor. From these optimal TDs upper limits on $ZT$ and power factor
are obtained; the maximum $ZT$ scales with $\Sigma_{\rm max} T /\kappa_l$,
where $\Sigma_{\rm max}$ is the TD magnitude and $\kappa_l$ is the lattice
thermal conductivity. These results help establish practical upper limits on
the performance of TE materials and provide target TDs to guide band/scattering
engineering strategies.

###Conventional Half-Heusler Alloys Advance State-of-the-Art Thermoelectric Properties|Mousumi Mitra,Allen Benton,Md Sabbir Akhanda,Jie Qi,Mona Zebarjadi,David J. Singh,S. Joseph Poon###

Conventional Half-Heusler Alloys Advance State-of-the-Art Thermoelectric Properties. Half-Heusler (HH) phases have garnered much attention as thermally stable and
non-toxic thermoelectric materials for power conversion. The most studied
alloys to date utilize Hf, Zr, and Ti as the base components. These alloys can
achieve a moderate dimensionless figure of merit, ZT, near 1. Recent studies
have advanced the thermoelectric performance of HH alloys by employing
nanostructures and novel compositions to achieve larger ZT, reaching as high as
1.5. Herein, we report that traditional alloying techniques applied to the
conventional HfZr-based half-Heusler alloys can also lead to exceptional ZT.
Specifically, we present the well-studied p-type Hf0.3Zr0.7CoSn0.3Sb0.7,
previously reported to have a ZT~0.8, resonantly doped with less than 1 at. %
metallic Al on the Sn/Sb site, touting a remarkable ZT near 1.5 at 980 K. This
is achieved through a significant increase in power factor, by ~65%, and a
notable but smaller decrease in thermal conductivity, by ~13%, at high
temperatures. These favorable thermoelectric properties are discussed in terms
of a local anomaly in the density of states near the Fermi energy designed to
enhance the Seebeck coefficient, as revealed by first-principles calculations,
as well as the emergence of a highly heterogeneous grain structure that can
scatter phonons across different length scales, effectively suppressing the
thermal conductivity. Consequently, the effective mass is significantly
enhanced from ~ 7 to 10me within a single parabolic band model, consistent with
the result from first-principles calculations. The discovery of high ZT in a
commonly studied half-Heusler alloy through a conventional and non-complex
approach opens a new path for further discoveries in similar types of alloys.
Furthermore, it is reasonable to believe that the study will reinvigorate
effort in exploring high thermoelectric performance in conventional alloy
systems.

###Thermoelectric Performance of 2D Tellurium with Accumulation Contacts|Gang Qiu,Shouyuan Huang,Mauricio Segovia,Prabhu K. Venuthurumilli,Yixiu Wang,Wenzhuo Wu,Xianfan Xu,Peide D. Ye###

Thermoelectric Performance of 2D Tellurium with Accumulation Contacts. Tellurium (Te) is an intrinsically p-type doped narrow bandgap semiconductor
with excellent electrical conductivity and low thermal conductivity. Bulk
trigonal Te has been theoretically predicted and experimentally demonstrated to
be an outstanding thermoelectric material with high value of thermoelectric
figure-of-merit ZT. In view of the recent progress in developing synthesis
route of two-dimensional (2D) tellurium thin films as well as the growing trend
of exploiting nanostructures as thermoelectric devices, here for the first time
we report excellent thermoelectric performance of tellurium nanofilms, with
room temperature power factor of 31.7 {\mu}Wcm-1K-2 and ZT value of 0.63. To
further enhance the efficiency of harvesting thermoelectric power in nanofilm
devices, thermoelectrical current mapping was performed with a laser as a
heating source, and we found high work function metals such as palladium can
form rare accumulation-type metal-to-semiconductor contacts to 2D Te, which
allows thermoelectrically generated carriers to be collected more efficiently.
High-performance thermoelectric 2D Te devices have broad applications as energy
harvesting devices or nanoscale Peltier coolers in microsystems.

###Rapid one-step synthesis and compaction of high-performance n-type Mg3Sb2 thermoelectrics|Jiawei Zhang,Lirong Song,Bo Brummerstedt Iversen###

Rapid one-step synthesis and compaction of high-performance n-type Mg3Sb2 thermoelectrics. n-type Mg3Sb2-based compounds are emerging as a promising class of low-cost
thermoelectric materials due to their extraordinary performance at low and
intermediate temperatures. However, so far high thermoelectric performance has
merely been reported in n-type Mg3Sb2-Mg3Bi2 alloys with a large amount of Bi.
Moreover, current synthesis methods of n-type Mg3Sb2 bulk thermoelectrics
involve multi-step processes that are time- and energy-consuming. Here we
report a fast and straightforward approach to fabricate n-type Mg3Sb2
thermoelectrics using spark plasma sintering, which combines the synthesis and
compaction in one step. Using this method, we achieve a high thermoelectric
figure of merit zT of ~0.4-1.5 at 300-725 K in n-type (Sc, Te)-doped Mg3Sb2
without alloying with Mg3Bi2. In comparison with the currently reported
synthesis methods, the complexity, process time, and cost of the new method are
significantly reduced. This work demonstrates a simple, low-cost route for the
potential large-scale production of n-type Mg3Sb2 thermoelectrics.

###Understanding and Designing the Spin-Driven Thermoelectrics|Md Mobarak Hossain Polash,Duncan Moseley,Junjie Zhang,Raphael P. Hermann,Daryoosh Vashaee###

Understanding and Designing the Spin-Driven Thermoelectrics. While the thermoelectric materials progress based on the engineering of
electronic and phononic characteristics is reaching a plateau, adding the spin
degree of freedom has the potential to open a new landscape for alternative
thermoelectric materials. Here we present the concepts, current understanding,
and guidelines for designing spin-driven thermoelectrics. We show that the
interplay between the spin and heat currents in entropy transport via charge
carriers can offer a strategic path to enhance the electronic thermopower. The
classical antiferromagnetic semiconductor manganese telluride (MnTe) is chosen
as the case study due to its significant spin-mediated thermoelectric
properties. We show that although the spin-disorder scattering reduces the
carrier mobility in magnetic materials, spin entropy, magnon, and paramagnon
carrier drags can dominate over and significantly enhance the thermoelectric
power factor and hence zT. Finally, several guidelines are drawn based on the
current understandings for designing high-performance spin-driven
thermoelectric materials.

###High temperature thermoelectric response of double-doped SrTiO$_3$ epitaxial films|Jayakanth Ravichandran,Wolter Siemons,Dong-wook Oh,Justin T. Kardel,Arvind Chari,Herman Heijmerikx,Matthew L. Scullin,Arun Majumdar,Ramamoorthy Ramesh,David G. Cahill###

High temperature thermoelectric response of double-doped SrTiO$_3$ epitaxial films. SrTiO$_3$ is a promising $n$-type oxide semiconductor for thermoelectric
energy conversion. Epitaxial thin films of SrTiO$_3$ doped with both La and
oxygen vacancies have been synthesized by pulsed laser deposition (PLD). The
thermoelectric and galvanomagnetic properties of these films have been
characterized at temperatures ranging from 300 K to 900 K and are typical of a
doped semiconductor. Thermopower values of double-doped films are comparable to
previous studies of La doped single crystals at similar carrier concentrations.
The highest thermoelectric figure of merit ($ZT$) was measured to be 0.28 at
873 K at a carrier concentration of $2.5\times10^{21}$ cm$^{-3}$.

###Superconducting and thermoelectric properties of new layered Superconductor Bi4O4S3|S. G. Tan,L. J. Li,Y. Liu,P. Tong,B. C. Zhao,W. J. Lu,Y. P. Sun###

Superconducting and thermoelectric properties of new layered Superconductor Bi4O4S3. Polycrystalline sample of the new layered superconductor Bi4O4S3 is
successfully synthesized by solid-state reaction method by using Bi, S and
Bi2O3 powders with one step reaction. The superconducting transition
temperature (Tconset=4.5 K), the zero resistance transition temperature
(Tc0=4.07 K) and the diamagnetic transition temperature (4.02 K at H=10 Oe)
were confirmed by electrical transport and magnetic measurements. Also, our
results indicate a typical type II-superconductor behavior. In addition, a
large thermoelectric effect was observed with a dimensionless thermoelectric
figure of merit (ZT) of about 0.03 at 300K, indicating Bi4O4S3 can be a
potential thermoelectric material.

###Estimates of the thermal conductivity and the thermoelectric properties of PbTiO$_3$ from first principles|Anindya Roy###

Estimates of the thermal conductivity and the thermoelectric properties of PbTiO$_3$ from first principles. The lattice thermal conductivity ($\kappa_{\rm L}$) of PbTiO$_3$ (PTO) is
estimated using a combination of {\em ab initio} calculations and semiclassical
Boltzmann transport equation. The computed $\kappa_{\rm L}$ is remarkably low,
nearly comparable with the $\kappa_{\rm L}$ of good thermoelectric materials
such as PbTe. In addition, a semiclassical analysis of the electronic transport
quantities is presented, which suggests excellent thermoelectric properties,
with a figure of merit $zT$ well over 1 for a wide range of temperature. For
thermoelectric applications, the $\kappa_{\rm L}$ could be further reduced by
utilizing different morphologies and compositions.

###Thermoelectric properties of new Bi-chalcogenide layered compounds|Yoshikazu Mizuguchi,Atsuhiro Nishida,Atsushi Omachi,Osuke Miura###

Thermoelectric properties of new Bi-chalcogenide layered compounds. The layered Bi-chalcogenide compounds have been drawing much attention as a
new layered superconductor family since 2012. Due to the rich variation of
crystal structure and constituent elements, the development of new physics and
chemistry of the layered Bi-chalcogenide family and its applications as
functional materials have been expected. Recently, it was revealed that the
layered Bi chalcogenides can show a relatively high thermoelectric performance
(ZT = 0.36 in LaOBiSSe at ~650 K). Here, we show the crystal structure
variation of the Bi-chalcogenide family and their thermoelectric properties.
Finally, the possible strategies for enhancing the thermoelectric performance
are discussed on the basis of the experimental and the theoretical facts
reviewed here.

###Anomalous transport and thermoelectric performances of CuAgSe compounds|A. J. Hong,L. Li,H. X. Zhu,X. H. Zhou,Q. Y. He,W. S. Liu,Z. B. Yan,J. M. Liu,Z. F. Ren###

Anomalous transport and thermoelectric performances of CuAgSe compounds. The copper silver selenide has two phases: the low-temperature semimetal
phase ({\alpha}-CuAgSe) and high-temperature phonon-glass superionic phase
(\b{eta}-CuAgSe). In this work, the electric transport and thermoelectric
properties of the two phases are investigated. It is revealed that the
\b{eta}-CuAgSe is a p-type semiconductor and exhibits low thermal conductivity
while the {\alpha}-CuAgSe shows metallic conduction with dominant n-type
carriers and low electrical resistivity. The thermoelectric figure of merit zT
of the polycrystalline \b{eta}-CuAgSe at 623 K is ~0.95, suggesting that
superionic CuAgSe can be a promising thermoelectric candidate in the
intermediate temperature range.

###Thermoelectric transport through Majorana bound states and violation of Wiedemann-Franz law|J. P. Ramos-Andrade,O. Ávalos-Ovando,P. A. Orellana,S. E. Ulloa###

Thermoelectric transport through Majorana bound states and violation of Wiedemann-Franz law. We study features of the thermoelectric transport through a Kitaev chain
hosting Majorana bound states (MBS) at its ends. We describe the behavior of
the Seebeck coefficient and the ZT figure of merit for two different
configurations between MBS and normal current leads. We find an important
violation of the Wiedemann-Franz law in one of these geometries, leading to
sizeable values of the thermoelectric efficiency over a narrow window in
chemical potential away from neutrality. These findings could lead to
interesting thermoelectric-based MBSs detection devices, via measurements of
the Seebeck coefficient and figure of merit.

###Feedback-type thermoelectric effect in correlated solids|Yugo Onishi,Naoto Nagaosa###

Feedback-type thermoelectric effect in correlated solids. A new thermoelectric effect mechanism inspired by an autonomous Maxwell's
demon [P. Strasberg, G. Schaller, T. Brandes, and M. Esposito, Phys. Rev. Lett.
110, 040601 (2013)] is proposed. In contrast to the former work where a model
for microscopic systems is proposed, a specific model for the thermoelectric
effect in solid is formulated and its response to the electric field and
temperature gradient is calculated in the framework of stochastic
thermodynamics. The results show that relatively high $ZT$, which represents
efficiency of the thermoelectric material, can be achieved within a range of
realistic parameters.

###Biaxial strain tuned thermoelectric properties in monolayer $\mathrm{PtSe_2}$|San-Dong Guo,Lun Zhang###

Biaxial strain tuned thermoelectric properties in monolayer $\mathrm{PtSe_2}$. Strain engineering is a very effective method to tune electronic, optical,
topological and thermoelectric properties of materials. In this work, we
systematically study biaxial strain dependence of electronic structures and
thermoelectric properties (both electron and phonon parts) of monolayer
$\mathrm{PtSe_2}$ with generalized gradient approximation (GGA) plus spin-orbit
coupling (SOC) for electron part and GGA for phonon part. Calculated results
show that compressive or tensile strain can induce conduction band minimum
(CBM) or valence band maximum (VBM) transition, which produces important
effects on Seebeck coefficient. It is found that compressive or tensile strain
can induce significantly enhanced n- or p-type Seebeck coefficient at the
critical strain of CBM or VBM transition, which can be explained by
strain-induced band convergence. Another essential strain effect is that
tensile strain can produce significantly reduced lattice thermal conductivity,
and the room temperature lattice thermal conductivity at the strain of -4.02\%
can decrease by about 60\% compared to unstrained one, which is very favorable
for high $ZT$. To estimate efficiency of thermoelectric conversion, the figure
of merit $ZT$ can be obtained by empirical scattering time $\tau$. Calculated
$ZT$ values show that strain indeed is a very effective strategy to achieve
enhanced thermoelectric properties, especially for p-type doping. Tuning
thermoelectric properties with strain also can be applied to other
semiconducting transition-metal dichalcogenide monolayers $\mathrm{MX_2}$
(M=Zr, Hf, Mo, W and Pt; X=S, Se and Te).

###Optimizing isotope and vacancy engineering in graphene ribbons to enhance the thermoelectric performance without degrading the electronic properties|Van-Truong Tran,Jérôme Saint-Martin,Philippe Dollfus,Sebastian Volz###

Optimizing isotope and vacancy engineering in graphene ribbons to enhance the thermoelectric performance without degrading the electronic properties. The enhancement of thermoelectric figure of merit ZT requires to either
increase the power factor or reduce the phonon conductance, or even both. In
graphene, the high phonon thermal conductivity is the main factor limiting the
thermoelectric conversion. The common strategy to enhance ZT is therefore to
introduce phonon scatterers to suppress the phonon conductance while retaining
high electrical conductance and Seebeck coefficient. Although thermoelectric
performance is eventually enhanced, all studies based on this strategy show a
significant reduction of the electrical conductance, most often leading to a
lower electronic performance. In this study we show that appropriate sources of
disorder, including isotopes and vacancies at lowest electron density
positions, can be used as phonon scatterers to reduce the phonon conductance in
graphene ribbons without degrading the electrical conductance, particularly in
the low-energy region which is the most important range for device operation.
By means of atomistic calculations using semi-empirical Tight-Binding and Force
Constant models in combination with Non-Equilibrium Green function formalism,
we show that the natural electronic properties of graphene ribbons can be fully
preserved while their thermoelectric efficiency is strongly enhanced. For
ribbons of width M = 5 dimer lines, room-temperature ZT is enhanced from less
than 0.26 for defect-free ribbons to more than 2.5. This study is likely to set
the milestones of a new generation of nano-devices with dual electronic,
thermoelectric functionalities.

###Lateral transition metal dichalcogenide heterostructures for high efficiency thermoelectric devices|Sathwik Bharadwaj,Ashwin Ramasubramaniam,L. R. Ram-Mohan###

Lateral transition metal dichalcogenide heterostructures for high efficiency thermoelectric devices. Increasing demands for renewable sources of energy has been a major driving
force for developing efficient thermoelectric materials. Two-dimensional (2D)
transition-metal dichalcogenides (TMDC) have emerged as promising candidates
for thermoelectric applications due to their large effective mass and low
thermal conductivity. In this article, we study the thermoelectric performance
of lateral TMDC heterostructures within a multiscale quantum transport
framework. Both $n$-type and $p$-type lateral heterostructures are considered
for all possible combinations of semiconducting TMDCs: MoS$_2$, MoSe$_2$,
WS$_2$, and WSe$_2$. The band alignment between these materials is found to
play a crucial in enhancing the thermoelectric figure-of-merit ($ZT$) and power
factor far beyond those of pristine TMDCs. In particular, we show that the
room-temperature $ZT$ value of $n$-type WS$_2$ with WSe$_2$ triangular
inclusions, is five times larger than the pristine WS$_2$ monolayer. $p$-type
MoSe$_2$ with WSe$_2$ inclusions is also shown to have a room-temperature $ZT$
value about two times larger than the pristine MoSe$_2$ monolayer. The peak
power factor values calculated here, are the highest reported amongst gapped 2D
monolayers at room temperature. Hence, 2D lateral TMDC heterostructures open
new avenues to develop ultra-efficient, planar thermoelectric devices.

###Significant suppression of thermal conductivity in FeSb2 by Te doping|Kefeng Wang,Rongwei Hu,C. Petrovic###

Significant suppression of thermal conductivity in FeSb2 by Te doping. Kondo insulator like material FeSb2 was found to exhibit colossal Seebeck
coefficient. It would have had huge potential in thermoelectric applications in
cryogenic temperature range if it had not been for the large thermal
conductivity. Here we studied the influence of Te doping at Sb site on thermal
conductivity and thermoelectric effect in high quality single crystals.
Surprisingly, only 5% Te doping suppresses thermal conductivity by two orders
of magnitude, which may be attributed to the substitution disorder. Te doping
also results in transition from an semiconductor to a metal. Consequently
thermoelectric figure of merit (ZT ? 0:05) in Fe(Sb0:9Te0:1)2 at ? 100K was
enhanced by about one order of magnitude when compared to ZT < 0:005 in undoped
FeSb2.

###Large enhancement of thermoelectric effects in a double quantum dot system due to interference and Coulomb correlation phenomena|Piotr Trocha,Józef Barnaś###

Large enhancement of thermoelectric effects in a double quantum dot system due to interference and Coulomb correlation phenomena. Thermoelectric effects in a double quantum dot system coupled to external
magnetic/nonmagnetic leads are investigated theoretically. The basic
thermoelectric transport characteristics, like thermopower, electronic
contribution to heat conductance, and the corresponding figure of merit, have
been calculated in terms of the linear response theory and Green function
formalism in the Hartree-Fock approximation for Coulomb interactions. An
enhancement of the thermal efficiency (figure of merit ZT) due to Coulomb
blockade has been found. The magnitude of ZT is further considerably enhanced
by quantum interference effects. Both the Coulomb correlations and interference
effects lead to strong violation of the Wiedemann-Franz law. The influence of
spin-dependent transport and spin bias on the thermoelectric effects
(especially on Seebeck and spin Seebeck effects) is also analyzed.

###Thermodynamic and Thermoelectric Properties of CoFeYGe (Y= Ti, Cr) Quaternary Heusler Alloys: First Principle Calculations|Raad Haleoot,Bothina Hamad###

Thermodynamic and Thermoelectric Properties of CoFeYGe (Y= Ti, Cr) Quaternary Heusler Alloys: First Principle Calculations. Utilizing a material in thermoelectric applications requires a mechanical,
thermal, and lattice stability as well a high figure of merit (ZT). In this
work, we present the structural, electronic, magnetic, mechanical,
thermodynamic, dynamic, and thermoelectric properties of CoFeYGe (Y = Ti, Cr)
quaternary Heusler compounds using the density functional theory (DFT). The
calculated mechanical properties and phonon dispersions reveal that the
structures of these compounds are stable. Both CoFeCrGe and CoFeTiGe compounds
show a ferromagnetic and ferrimagnetic half-metallic behavior with band gaps of
0.41 and 0.38 eV, respectively. The lattice thermal conductivity (\k{appa}L)
exhibits low values that reach 3.01 W/(m.K) (3.47 W/(m.K)) for CoFeCrGe
(CoFeTiGe) at 1100 K. The optical phonon modes have a large contribution of
60.2% (70.9 %) to \k{appa}L value for CoFeCrGe (CoFeTiGe). High ZT values of
0.71 and 0.65 were obtained for CoFeCrGe and CoFeTiGe, respectively. Based on
our calculations, CoFeCrGe and CoFeTiGe combine both good spintronic and
thermoelectric behaviors that may be used in spin injection applications.

###Ultrahigh thermoelectric performance of Janus α-STe2 and α-SeTe2 monolayers|Gang Liu,Aiqing Guo,Fengli Cao,Weiwei Ju,Zhaowu Wang,Hui Wang,Guo-Ling Li,Zhibin Gao###

Ultrahigh thermoelectric performance of Janus α-STe2 and α-SeTe2 monolayers. Combined with first-principles calculations and semiclassical Boltzmann
transport theory, Janus {\alpha}-STe2 and {\alpha}-SeTe2 monolayers are
investigated systematically. Janus {\alpha}-STe2 and {\alpha}-SeTe2 monolayers
are indirect semiconductors with band gaps of 1.20 and 0.96 eV. It is found
they possess ultrahigh figure of merit (ZT) values of 3.9 and 4.4 at 500 K,
much higher than that of the pristine {\alpha}-Te monolayer (2.8). The higher
ZT originates from Janus structures reduce lattice thermal conductivities
remarkably compared with pristine {\alpha}-Te monolayer. The much higher phonon
anharmonicity in Janus monolayers leads to significant lower lattice thermal
conductivity. It is also found electronic thermal conductivity can play an
important role in thermoelectric efficiency for the materials with quite low
lattice thermal conductivity. This work suggests the potential applications of
Janus {\alpha}-STe2 and {\alpha}-SeTe2 monolayers as thermoelectric materials
and highlights Janus structure as an effective way to enhance thermoelectric
performance.

###Analysis of Thermoelectric Properties of Scaled Silicon Nanowires Using an Atomistic Tight-Binding Model|Neophytos Neophytou,Martin Wagner,Hans Kosina,Siegfried Selberherr###

Analysis of Thermoelectric Properties of Scaled Silicon Nanowires Using an Atomistic Tight-Binding Model. Low dimensional materials provide the possibility of improved thermoelectric
performance due to the additional length scale degree of freedom for
engineering their electronic and thermal properties. As a result of suppressed
phonon conduction, large improvements on the thermoelectric figure of merit,
ZT, have been recently reported in nanostructures, compared to the raw
materials' ZT values. In addition, low dimensionality can improve a device's
power factor, offering an additional enhancement in ZT. In this work the
atomistic sp3d5s*-spin-orbit-coupled tight-binding model is used to calculate
the electronic structure of silicon nanowires (NWs). The Landauer formalism is
applied to calculate an upper limit for the electrical conductivity, the
Seebeck coefficient, and the power factor. We examine n-type and p-type
nanowires of diameters from 3nm to 12nm, in [100], [110], and [111] transport
orientations at different doping concentrations. Using experimental values for
the lattice thermal conductivity in nanowires, an upper limit for ZT is
computed. We find that at room temperature, scaling the diameter below 7nm can
at most double the power factor and enhance ZT. In some cases, however, scaling
does not enhance the performance at all. Orientations, geometries, and subband
engineering techniques for optimized designs are discussed.

###Superhigh thermoelectric figure of merit in silver halides AgCl and AgBr from first principles|Xiuxian Yang,Zhenhong Dai,Yinchang Zhao,Sheng Meng###

Superhigh thermoelectric figure of merit in silver halides AgCl and AgBr from first principles. Searching for the high-performance thermoelectric (TE) materials has always
been a long-held dream in the thermoelectricity field. Recently, it is found in
experiments that the largest figure of merit ZT of $2.6$ can be reached in SnSe
crystals at 923 K and Cu2Se sample at 850 K, which arouses the enormous
interest of seeking high-ZT materials. Based on first-principle calculations
and Boltzman transport equation (BTE), we report in this letter that silver
halides (AgCl and AgBr) in rocksalt structure have excellent TE performances. A
superhigh ZT of about 7.0 at mid-temperature (~600K) is obtained in the p-type
doped AgCl and AgBr crystals, which far exceeds the ZT values of all current
bulk TE materials. This record-breaking ZT value is attributed to the ultralow
intrinsic lattice thermal conductivity \kappa_L (e.g. \kappa_L ~0.10 and 0.09
Wm^{-1}K^{-1} for AgCl and AgBr at 600 K, respectively). Our results may be a
feat that could revolutionize the field of the heat energy conversion.

###Non-universal Scaling of Thermoelectric Efficiency in 3D and 2D Thermoelectric Semiconductors|Kevin Octavian,Eddwi H. Hasdeo###

Non-universal Scaling of Thermoelectric Efficiency in 3D and 2D Thermoelectric Semiconductors. We performed the first-principles calculation on common thermoelectric
semiconductors $\rm Bi_2Te_3$, $\rm Bi_2Se_3$, $\rm SiGe$, and $\rm PbTe$ in
bulk three-dimension (3D) and two-dimension (2D). We found that miniaturization
of materials does not generally increase the thermoelectric figure of merit
($ZT$) according to the Hicks and Dresselhaus (HD) theory. For example, $ZT$
values of 2D $ \rm PbTe$ (0.32) and 2D $ \rm SiGe$ (0.04) are smaller than
their 3D counterparts (0.49 and 0.09, respectively). Meanwhile, the $ZT$ values
of 2D $\rm Bi_2Te_3$ (0.57) and 2D $\rm Bi_2Se_3$ (0.43) are larger than the
bulks (0.54 and 0.18, respectively), which agree with HD theory. The HD theory
breakdown occurs because the band gap and band flatness of the materials change
upon dimensional reduction. We found that flat bands give a larger electrical
conductivity ($\sigma$) and electronic thermal conductivity ($\kappa_{el}$) in
3D materials, and smaller values in 2D materials. In all cases, maximum $ZT$
values increase proportionally with the band gap and saturate for the band gap
above $10\ k_BT$. The 2D $Bi_2Te_3$ and $Bi_2Se_3$ obtain a higher $ZT$ due to
the flat corrugated bands and narrow peaks in their DOS. Meanwhile, the 2D PbTe
violates HD theory due to the flatter bands it exhibits, while 2D SiGe
possesses a small gap Dirac-cone band.

###Microscopic origin of the excellent thermoelectric performance in n-doped SnSe|Anderson S. Chaves,Daniel T. Larson,Efthimios Kaxiras,Alex Antonelli###

Microscopic origin of the excellent thermoelectric performance in n-doped SnSe. Excellent thermoelectric performance in the out-of-layer n-doped SnSe has
been observed experimentally (Chang et al., Science 360, 778-783 (2018)).
However, a first-principles investigation of the dominant scattering mechanisms
governing all thermoelectric transport properties is lacking. In the present
work, by applying extensive first-principles calculations of electron-phonon
coupling associated with the calculation of the scattering by ionized
impurities, we investigate the reasons behind the superior figure of merit as
well as the enhancement of zT above 600 K in n-doped out-of-layer SnSe, as
compared to p-doped SnSe with similar carrier densities. For the n-doped case,
the relaxation time is dominated by ionized impurity scattering and increases
with temperature, a feature that maintains the power factor at high values at
higher temperatures and simultaneously causes the carrier thermal conductivity
at zero electric current (k_el) to decrease faster for higher temperatures,
leading to an ultrahigh-zT = 3.1 at 807 K. We rationalize the roles played by
k_el and k^0 (the thermal conductivity due to carrier transport under
isoelectrochemical conditions) in the determination of zT. Our results show the
ratio between k^0 and the lattice thermal conductivity indeed corresponds to
the upper limit for zT, whereas the difference between calculated zT and the
upper limit is proportional to k_el.

###Thermoelectric properties of orthorhombic group IV-VI monolayers from the first-principles calculations|San-Dong Guo###

Thermoelectric properties of orthorhombic group IV-VI monolayers from the first-principles calculations. Two-dimensional (2D) materials may have potential applications in
thermoelectric devices. In this work, we systematically investigate the
thermoelectric properties of orthorhombic group IV-VI monolayers $\mathrm{AB}$
(A=Ge and Sn; B=S and Se) by the first-principles calculations and
semiclassical Boltzmann transport theory. The spin-orbit coupling (SOC) is
included to investigate their electronic transport, which produces observable
effects on power factor, especially for n-type doping. According to calculated
$ZT$, the four monolayers exhibit diverse anisotropic thermoelectric
properties, although they have similar hinge-like crystal structure. The GeS
along zigzag and armchair directions shows the strongest anisotropy, while SnS
and SnSe show mostly isotropic efficiency of thermoelectric conversion, which
can be understood by the strength of anisotropy of their respective power
factor, electronic and lattice thermal conductivities. Calculated results show
that $ZT$ for different carriers of n- and p-type has little difference for
GeS, SnS and SnSe. It is found that GeSe, SnS and SnSe show better
thermoelectric performance compared to GeS in n-type doping, and SnS and SnSe
exhibit higher efficiency of thermoelectric conversion in p-type doping.
Compared to a lot of 2D materials, orthorhombic group IV-VI monolayers
$\mathrm{AB}$ (A=Ge and Sn; B=S and Se) may possess better thermoelectric
performance due to higher power factor and lower thermal conductivity. Our work
would be beneficial to further experimental study.

###Thermoelectric properties of in-plane $90^0$-bent graphene nanoribbons with nanopores|Van-Truong Tran,Alessandro Cresti###

Thermoelectric properties of in-plane $90^0$-bent graphene nanoribbons with nanopores. We study the thermoelectric performance of $90^0$-bent graphene nanoribbons
containing nanopores for optimized design of multiple functional circuits
including thermoelectric generators. We show that the thermal conductance of
the $90^0$-bent ribbons is lower from few times to an order of magnitude
compared to that of pristine armchair and zigzag straight ribbons.
Consequently, the thermoelectric performance of the bent ribbons is better than
its straight ribbon counterparts, in particular at high temperatures above 500
K. More importantly, the introduction of nanopores is demonstrated to strongly
enhance their thermoelectric capacity. At 500 K, the figure of merit ZT
increases by more than 160% (from 0.39 without pores to 0.64) with 3 nanopores
incorporated, and by more than 200% (up to 0.88) when 24 nanopores are
introduced. ZT 1 can be achieved at a temperature of about 1000 K. In addition,
the thermoelectric performance is shown to be further improved by adopting
asymmetrical leads. This study demonstrates that $90^0$-bent ribbons with
nanopores have decent thermoelectric performance for a wide range of
temperatures and may find application as efficient thermoelectric converters.

###Universal Limits of Thermopower and Figure of Merit from Transport Energy Statistics|Peter Zahn###

Universal Limits of Thermopower and Figure of Merit from Transport Energy Statistics. The search for new thermoelectric materials aims at improving their power and
efficiency, as expressed by thermopower $S$ and figure of merit $ZT$. By
considering a very general transport spectral function $W(E)$, expressions for
$S$ and $ZT$ can be derived, which contain the statistical weights of an
effective distribution function only, see Refs.
\cite{eltschka16,mahan96,matveev00}.
  We assumption of a Lorentzian shape with width $k_BT$ resulting from the
electron-phonon coupling allows to estimate an upper limit of $S$ and $ZT$
independent on the microscopic mechanisms of the transport process. A simple
estimate for an upper limit of the thermopwer $S$ is derived from {} formula.
It is given by 3 times the unit of the thermopower $k_b/e$ which is about
250~$\mu V/K$.
  We consider different systems which represent the general features of the
electronic structure of thermoelectric relevant materials very well. The
transport integrals were evaluated varying the band gap size and the chemical
potential position. For all cases upper limits for both, the thermopower and
the figure of merit, are obtained. The universal limit of $|S|$ is given by
1.88 in units of $k_B/e$, which is about 160~$\mu V/K$. The universal limit for
$ZT$ is obtained by about 1.11, which is in good agreement with available
thermoelectric systems and devices.

###Giant thermopower and figure of merit in single-molecule devices|C. M. Finch,V. M. García-Suárez,C. J. Lambert###

Giant thermopower and figure of merit in single-molecule devices. We present a study of the thermopower $S$ and the dimensionless figure of
merit $ZT$ in molecules sandwiched between gold electrodes. We show that for
molecules with side groups, the shape of the transmission coefficient can be
dramatically modified by Fano resonances near the Fermi energy, which can be
tuned to produce huge increases in $S$ and $ZT$. This shows that molecules
exhibiting Fano resonances have a high efficiency of thermoelectric cooling
which is not present for conventional un-gated molecules with only delocalized
states along their backbone.

###Enhancement of thermoelectric figure-of-merit of Graphene upon BN-doping and sample length reduction|Ransell D'Souza,Sugata Mukherjee###

Enhancement of thermoelectric figure-of-merit of Graphene upon BN-doping and sample length reduction. Using first-principles density functional perturbation theory based
calculations of length-dependent lattice thermal conductivity (\k{appa} L ) and
using our previously calculated results (Phys Rev B 95 085435 (2017)) of
electrical transport, we report results of thermoelectric figure-of-merit (ZT )
of monolayer and bilayer Graphene. We find nearly ten-fold increase in ZT for
the graphene sample doped with boron nitride and reduced sample length. We also
compare \k{appa} L calculated using the iterative real space method with
conventional analytical Callaway-Klemens method and obtain the flexural (ZA)
phonon modes to be dominant in thermal transport unlike in the latter method.
Our calculations are in good agreement with available experimental data.

###Resonant thermoelectric transport in atomic chains with Fano defects|J. Eduardo González,Vicenta Sánchez,Chumin Wang###

Resonant thermoelectric transport in atomic chains with Fano defects. Atomic clusters attached to a low-dimensional system, called Fano defects,
produce rich wave interferences. In this work, we analytically found an
enhanced thermoelectric figure-of-merit (ZT) in periodic atomic chains with
Fano defects, compared to those without such defects. We further study
self-assembled DNA-like systems with periodic and quasiperiodically placed Fano
defects by using a real-space renormalization method developed for the
Kubo-Greenwood formula, in which tight-binding and Born models are respectively
used for the electric and lattice thermal conductivities. The results reveal
that the quasiperiodicity could be another ZT-improving factor, whose
long-range disorder inhibits low-frequency acoustic phonons insensitive to
local defects.

###Enhanced thermoelectric response in the fractional quantum Hall effect|Pablo Roura-Bas,Liliana Arrachea,Eduardo Fradkin###

Enhanced thermoelectric response in the fractional quantum Hall effect. We study the linear thermoelectric response of a quantum dot embedded in a
constriction of a quantum Hall bar with fractional filling factors nu=1/m
within Laughlin series. We calculate the figure of merit ZT for the maximum
efficiency at a fixed temperature difference. We find a significant enhancement
of this quantity in the fractional filling in relation to the integer-filling
case, which is a direct consequence of the fractionalization of the electron in
the fractional quantum Hall state. We present simple theoretical expressions
for the Onsager coefficients at low temperatures, which explicitly show that ZT
and the Seebeck coefficient increase with m.

###Enhanced thermoelectric performance of twisted bilayer graphene nanoribbons junction|Shuo Deng,Xiang Cai,Yan Zhang,Lijie Li###

Enhanced thermoelectric performance of twisted bilayer graphene nanoribbons junction. We investigate the electron transport and thermoelectric property of twisted
bilayer graphene nanoribbon junction (TBGNRJ) in $0^o$, $21.8^o$, $38.2^o$ and
$60^o$ rotation angles by first principles calculation with Landauer-Buttiker
and Boltzmann theories. It is found that TBGNRJs exhibit negative differential
resistance (NDR) in $21.8^o$ and $38.2^o$ rotation angles under $\pm$ 0.2 V
bias voltage. More importantly, three peak ZT values of 2.0, 2.7 and 6.1 can be
achieved in the $21.8^o$ rotation angle at 300K. The outstanding ZT values of
TBGNRJs are interpreted as the combination of the reduced thermal conductivity
and enhanced electrical conductivity at optimized angles.

###Thermoelectric properties of semiconducting materials with parabolic and pudding-mold band structures|Jyesta M. Adhidewata,Ahmad R. T. Nugraha,Eddwi H. Hasdeo,Patrice Estelle,Bobby E. Gunara###

Thermoelectric properties of semiconducting materials with parabolic and pudding-mold band structures. We theoretically investigate the thermoelectric properties of semiconducting
(gapped) materials by varying the degrees of polynomials in their energy
dispersion relations, in which either the valence or conduction energy
dispersion depends on the wave vector raised to the power of two, four, and
six. The thermoelectric transport coefficients such as the Seebeck coefficient,
electrical conductivity, and thermal conductivity are calculated within the
linearized Boltzmann transport theory combined with the relaxation time
approximation. We consider various effects such as band gaps, dimensionalities,
and dispersion powers to understand the conditions that can give the optimal
thermoelectric efficiency or figure of merit ($ZT$). Our calculations show that
the so-called pudding-mold band structure produces larger electrical and
thermal conductivities than the parabolic band, but no significant difference
is found in the Seebeck coefficients of the pudding-mold and parabolic bands.
Furthermore, we find that a high $ZT$ can be obtained by tuning the band gap of
the material to an optimum value simultaneously with breaking the band
symmetry. The largest $ZT$ is found in a combination of two-contrasting
polynomial powers in the dispersion relations of valence and conduction bands.
This band asymmetry also shifts the charge neutrality away from the undoped
level and allows optimal $ZT$ to be located at a smaller chemical potential.
With some reasonable values of thermal conductivity parameters, the maximum
$ZT$ for the bulk systems can be larger than 1, while for one-dimensional
systems it can even reach almost 4. We expect this work to trigger
high-throughput calculations for screening of potential thermoelectric
materials combining various polynomial powers in the energy dispersion
relations of semiconductors.

###Effect of Thermoelectric Cooling in Nanoscale Junctions|Yu-Shen Liu,Bailey C. Hsu,Yu-Chang Chen###

Effect of Thermoelectric Cooling in Nanoscale Junctions. We propose a thermoelectric cooling device based on an atomic-sized junction.
Using first-principles approaches, we investigate the working conditions and
the coefficient of performance (COP) of an atomic-scale electronic refrigerator
where the effects of phonon's thermal current and local heating are included.
It is observed that the functioning of the thermoelectric nano-refrigerator is
restricted to a narrow range of driving voltages. Compared with the bulk
thermoelectric system with the overwhelmingly irreversible Joule heating, the
4-Al atomic refrigerator has a higher efficiency than a bulk thermoelectric
refrigerator with the same $ZT$ due to suppressed local heating via the
quasi-ballistic electron transport and small driving voltages. Quantum nature
due to the size minimization offered by atomic-level control of properties
facilitates electron cooling beyond the expectation of the conventional
thermoelectric device theory.

###Quaternary compounds Ag2XYSe4 (X=Ba, Sr; Y=Sn, Ge) as novel potential thermoelectric materials|A. J. Hong,C. L. Yuan,J. M. Liu###

Quaternary compounds Ag2XYSe4 (X=Ba, Sr; Y=Sn, Ge) as novel potential thermoelectric materials. Experimental results have shown that the quaternary compound Cu2ZnSnSe4 is an
excellent thermoelectric material. This inspires us to seek the other
quaternary compounds with similar chemical formula to Cu2ZnSnSe4 as
thermoelectric materials. In this paper, we use the first-principle method to
systematically explore the electronic and phonon structures, mechanical,
thermal and thermoelectric properties of p- and n-type Ag2XYSe4 (X=Ba, Sr;
Y=Sn, Ge). It is found that the ZT maximum for n-type Ag2SrGeSe4 can reach up
to 1.22 at 900 K, and those for p-type Ag2SrSnSe4, Ag2SrGeSe4 and Ag2BaSnSe4
can reach up to 1.20, 1.13 and 1.12, respectively. Our work not only shows that
Ag2XYSe4 (X=Ba, Sr; Y=Sn, Ge) are a kind of potential thermoelectric materials,
but also can inspire more theoretical and experimental researches on
thermoelectric properties of quaternary compounds.

###Thermoelectric signatures of order-parameter symmetries in iron-based superconducting tunnel junctions|Claudio Guarcello,Alessandro Braggio,Francesco Giazotto,Roberta Citro###

Thermoelectric signatures of order-parameter symmetries in iron-based superconducting tunnel junctions. Thermoelectrical properties are frequently used to characterize the materials
and endow the free energy from wasted heat for useful purposes. Here, we show
that linear thermoelectric effects in tunnel junctions (TJs) with Fe-based
superconductors, not only address the dominance between particle and hole
states, but even provide information about the superconducting order parameter
symmetry. In particular, we observe that nodal order parameters present a
maximal thermoelectric effect at lower temperatures than for nodeless cases.
Finally, we show that superconducting TJs between iron-based and BCS
superconductors could provide a thermoelectric efficiency ZT exceeding 6 with a
linear Seebeck coefficient around $S\approx 800\;\mu\text{V/K}$ at a few
Kelvin. These results pave the way to novel thermoelectric machines based on
multi-band superconductors.

###Enhanced Thermoelectric Efficiency via Orthogonal Electrical and Thermal Conductances in Phosphorene|Ruixiang Fei,Alireza Faghaninia,Ryan Soklaski,Jia-An Yan,Cynthia Lo,Li Yang###

Enhanced Thermoelectric Efficiency via Orthogonal Electrical and Thermal Conductances in Phosphorene. Thermoelectric devices that utilize the Seebeck effect convert heat flow into
electrical energy and are highly desirable for the development of portable,
solid state, passively-powered electronic systems. The conversion efficiencies
of such devices are quantified by the dimensionless thermoelectric figure of
merit (ZT), which is proportional to the ratio of a device's electrical
conductance to its thermal conductance. High ZT (>2) has been achieved in
materials via all-scale hierarchical architecturing. This efficiency holds at
high temperatures (700K~900K) but quickly diminishes at lower temperatures. In
this paper, a recently-fabricated two-dimensional (2D) semiconductor called
phosphorene (monolayer black phosphorus) is assessed for its thermoelectric
capabilities. First-principles and model calculations reveal that phosphorene
possesses spatially-anisotropic electrical and thermal conductances. The
prominent electrical and thermal conducting directions are orthogonal to one
another, enhancing the ratio of these conductances. As a result, ZT can reach
2.5 (the criterion for commercial deployment) along the armchair direction of
phosphorene at T=500K and is greater than 1 even at room temperature given
moderate doping (~2 x 10^16 m-2). Ultimately, phosphorene stands out as an
environmentally sound thermoelectric material with unprecedented qualities:
intrinsically, it is a mechanically flexible material that converts heat energy
with high efficiency at low temperatures (~ 300K) - one whose performance does
not require any sophisticated engineering techniques.

###Impressive optoelectronic and thermoelectric properties of two-dimensional XI$_2$ (X=Sn, Si): a first principle study|Atanu Betal,Jayanta Bera,Satyajit Sahu###

Impressive optoelectronic and thermoelectric properties of two-dimensional XI$_2$ (X=Sn, Si): a first principle study. Two-dimensional (2D) metal halides have received more attention because of
their electronic and optoelectronic properties. Recently, researchers are
interested to investigate the thermoelectric properties of metal halide
monolayers because of their ultralow lattice conductivity, high Seebeck
coefficient and figure of merit. Here, we have investigated thermoelectric and
optoelectronic properties of XI$_2$ (X=Sn and Si) monolayers with the help of
density functional theory and Boltzmann transport equation. The structural
parameters have been optimized with relaxation of atomic positions. Excellent
thermoelectric and optical properties have been obtained for both SnI$_2$ and
SiI$_2$ monolayers. For SnI$_2$ an indirect bandgap of 2.06 eV was observed and
the absorption peak was found at 4.68 eV. For this the highest ZT value of 0.84
for p-type doping at 600K has been calculated. Similarly, for SiI$_2$ a
comparatively low indirect bandgap of 1.63 eV was observed, and the absorption
peak was obtained at 4.86 eV. The calculated ZT product for SiI$_2$ was 0.87 at
600K. Both the crystals having high absorbance and ZT value suggest that they
can be promising candidates for optoelectronic and thermoelectric devices.

###Effects of intervalley scatterings in thermoelectric performance of band-convergent antimonene|Yu Wu,Bowen Hou,Congcong Ma,Jiang Cao,Ying Chen,Zixuan Lu,Haodong Mei,Hezhu Shao,Yuanfeng Xu,Heyuan Zhu,Zhilai Fang,Rongjun Zhang,Hao Zhang###

Effects of intervalley scatterings in thermoelectric performance of band-convergent antimonene. The strategy of band convergence of multi-valley conduction bands or
multi-peak valence bands has been widely used to search or improve
thermoelectric materials. However, the phonon-assisted intervalley scatterings
due to multiple band degeneracy are usually neglected in the thermoelectric
community. In this work, we investigate the (thermo)electric properties of
non-polar monolayer $\beta$- and $\alpha$-antimonene considering full mode- and
momentum-resolved electron-phonon interactions. We also analyze thoroughly the
selection rules on electron-phonon matrix-elements using group-theory
arguments. Our calculations reveal strong intervalley scattering between the
nearly degenerate valley states in both $\beta$- and $\alpha$-antimonene, and
the commonly-used deformation potential approximation neglecting the dominant
intervalley scattering gives inaccurate estimations of the electron-phonon
scattering and thermoelectric transport properties. By considering full
electron-phonon interactions based on the rigid-band approximation, we find
that, the maximum value of the thermoelectric figure of merits $zT$ at room
temperature reduces to 0.37 in $\beta$-antimonene, by a factor of 5.7 comparing
to the value predicted based on the constant relaxation-time approximation
method. Our work not only provides an accurate prediction of the thermoelectric
performances of antimonenes that reveals the key role of intervalley
scatterings in determining the electronic part of zT, but also showcases a
computational framework for thermoelectric materials.

###Phonon Coupled Scattering Caused Ultralow Lattice Thermal Conductivity and Its Role in The Remarkable Thermoelectric Performance of Newly Predicted SiS2 and SiSe2 monolayers|Jayanta Bera,Atanu Betal,Satyajit Sahu###

Phonon Coupled Scattering Caused Ultralow Lattice Thermal Conductivity and Its Role in The Remarkable Thermoelectric Performance of Newly Predicted SiS2 and SiSe2 monolayers. For high efficiency thermoelectric power conversion not only improvement of
materials properties but also prediction and synthesis of new thermoelectric
materials is needed. Here we have carried out a systematic investigation on
thermoelectric performance of newly predicted two dimensional (2D)
semiconducting SiS2 and SiSe2 monolayers of group IVA-VIA family using density
functional theory (DFT) and Boltzmann transport equation (BTE). Our computed
values of lattice thermal conductivity (kph) are ultralow which result very
high thermoelectric figure of merit (ZT) value of 0.78 (0.80) at 500K in SiS2
(SiSe2) monolayer. The ultralow values of kph are attributed to phonon-phonon
coupling of acoustic and low frequency optical branches which leads to larger
scattering, low group velocity, smaller mean free path and shorter lifetime of
phonons. It is also found from our investigation that p-type doping is more
effective than n-type doping to get optimal power factor (PF) and ZT. Our
theoretical investigation suggests that newly predicted semiconducting SiS2 and
SiSe2 monolayers can be very promising thermoelectric materials for fabrication
of high efficiency thermoelectric power generator to convert wastage heat into
electricity.

###Realizing high Near-Room-Temperature Thermoelectric Performance in n-type Ag2Se through Rashba Effect and Entropy Engineering|Raju K Biswas,Swapan K Pati###

Realizing high Near-Room-Temperature Thermoelectric Performance in n-type Ag2Se through Rashba Effect and Entropy Engineering. Although there are enormous numbers of high-temperature thermoelectric
materials present, designing a near-room-temperature especially n-type
thermoelectric material with high zT is extremely challenging. Generally,
pristine Ag2Se exhibits unusual low thermal conductivity along with high
electrical conductivity and Seebeck coefficient, which leads to high
thermoelectric performance (n-type) at room temperature. Herein, we report a
pseudoternary phase, Ag2Se0.5Te0.25S0.25, which shows improved thermoelectric
performance (zT ~ 2.1 at 400 K). Density functional theory reveals that the
Rashba type of spin-dependent band spitting originated because of Te-doping,
enhancing carrier mobility. Using density functional perturbation theory, we
hereby realize that the intrinsic carrier mobility is not only controlled by
carrier effective mass, neither deformation potential theory, instead it is
substantially limited by longitudinal optical phonon scattering. In fact,
locally off-centered S atoms and rising configurational entropy via
substitution of Te and S atoms in Ag2Se significantly reduce the lattice
thermal conductivity (klat ~ 0.34 at 400 K). In order to accurately obtain
electrical as well as thermal transport coefficient, we adopt deformation
potential theory based on Boltzmann transport formalism. The combined
consequence of the Rashba effect coupled with configurational entropy
synergistically results in such high thermoelectric performance with the
development of new n-type thermoelectric material working at the
near-room-temperature regime.

###Optimizing thermal transport in the Falicov-Kimball model: binary-alloy picture|J. K. Freericks,V. Zlatic###

Optimizing thermal transport in the Falicov-Kimball model: binary-alloy picture. We analyze the thermal transport properties of the Falicov-Kimball model
concentrating on locating regions of parameter space where the thermoelectric
figure-of-merit ZT is large. We focus on high temperature for power generation
applications and low temperature for cooling applications. We constrain the
static particles (ions) to have a fixed concentration, and vary the conduction
electron concentration as in the binary-alloy picture of the Falicov-Kimball
model. We find a large region of parameter space with ZT>1 at high temperature
and we find a small region of parameter space with ZT>1 at low temperature for
correlated systems, but we believe inclusion of the lattice thermal
conductivity will greatly reduce the low-temperature figure-of-merit.

###Effects of interdot hopping and Coulomb blockade on the thermoelectric properties of serially coupled quantum dots|David M. -T. Kuo,Y. C. Chang###

Effects of interdot hopping and Coulomb blockade on the thermoelectric properties of serially coupled quantum dots. We have theoretically studied the thermoelectric properties of serially
coupled quantum dots (SCQD) embedded in an insulator matrix connected to
metallic electrodes. In the framework of Keldysh Green's function technique,
the Landauer formula of transmission factor is obtained by using the equation
of motion method. Based on such analytical expressions of charge and heat
currents, we calculate the electrical conductance, Seebeck coefficient,
electron thermal conductance and figure of merit (ZT) of SCQD in the linear
response regime. The effects of electron Coulomb interactions on the reduction
and enhancement of ZT are analyzed. We demonstrate that ZT is not a monotonic
increasing function of interdot electron hopping strength ($t_c$). We also show
that in the absence of phonon thermal conductance, SCQD can reach the Carnot
efficiency as $t_c$ approaches zero.

###Thermoelectricity of Wigner crystal in a periodic potential|O. V. Zhirov,D. L. Shepelyansky###

Thermoelectricity of Wigner crystal in a periodic potential. We study numerically the thermoelectricity of the classical Wigner crystal
placed in a periodic potential and being in contact with a thermal bath modeled
by the Langevin dynamics. At low temperatures the system has sliding and pinned
phases with the Aubry transition between them. We show that in the Aubry pinned
phase the dimensionless Seebeck coefficient can reach very high values of
several hundreds. At the same time the charge and thermal conductivity of
crystal drop significantly inside this phase. Still we find that the largest
values of $ZT$ factor are reached in the Aubry phase and for the studied
parameter range we obtain $ZT \leq 4.5$. We argue that this system can provide
an optimal regime for reaching high $ZT$ factors and realistic modeling of
thermoelecriticy. Possible experimental realizations of this model are
discussed.

###Thermoelectric properties of semiconductor nanowire networks|Oleksiy Roslyak,Andrei Piryatinski###

Thermoelectric properties of semiconductor nanowire networks. To examine thermoelectric (TE) properties of a semiconductor nanowire (NW)
network, we propose a theoretical approach mapping the TE network on a two-port
network. In contrast to a conventional single-port (i.e., resistor) network
model, our model allows for large scale calculations showing convergence of TE
figure of merit, $ZT$, with an increasing number of junctions. Using this
model, numerical simulations are performed for the Bi$_2$Te$_3$ branched
nanowire (BNW) and Cayley tree NW (CTNW) network. We find that the phonon
scattering at the network junctions plays a dominant role in enhancing the
network $ZT$. Specifically, disordered BNW and CTNW demonstrate an order of
magnitude higher ZT enhancement compared to their ordered counterparts.
Formation of preferential TE pathways in CTNW makes the network effectively
behave as its BNW counterpart. We provide formalism for simulating large scale
nanowire networks hinged upon experimentally measurable TE parameters of a
single T-junction.

###Enhanced thermoelectric performance of carbon nanotubes at elevated temperature|P. H. Jiang,H. J. Liu,D. D. Fan,L. Cheng,J. Wei,J. Zhang,J. H. Liang,J. Shi###

Enhanced thermoelectric performance of carbon nanotubes at elevated temperature. The electronic and transport properties of (10, 0) single-walled carbon
nanotube are studied by performing the first-principles calculations and
semi-classical Boltzmann theory. It is found that the (10, 0) tube exhibits
considerably large Seebeck coefficient and electrical conductivity which is
highly desirable for good thermoelectric materials. Together with the lattice
thermal conductivity predicted by non-equilibrium molecular dynamics
simulations, the room temperature ZT value of (10, 0) tube is estimated to be
0.15 for p-type carriers. Moreover, the ZT value exhibits strong temperature
dependence and can be reached to 0.77 at 1000 K. Such ZT value can be further
enhanced to as high as 1.7 by isotope substitution and chemisorptions of
hydrogen on the tube.

###Spin and charge caloritronics in bilayer graphene flakes with magnetic contacts|Leonor Chico,P. A. Orellana,L. Rosales,M. Pacheco###

Spin and charge caloritronics in bilayer graphene flakes with magnetic contacts. We investigate the coupling of spin and thermal currents as a means to rise
the thermoelectric efficiency of nanoscale graphene devices. We consider
nanostructures composed of overlapping graphene nanoribbons with ferromagnetic
contacts in different magnetic configurations. Our results show that the charge
Seebeck effect is greatly enhanced when the magnetic leads are in an
antiparallel configuration, due to the enlargement of the transport gap.
However, for the optimization of the charge figure of merit ZT it is better to
choose a parallel alignment of the magnetization in the leads, because the
electron-hole symmetry is broken in this magnetic configuration. We also obtain
the spin-dependent Seebeck coefficient and spin figure of merit. In fact, the
spin ZT can double its value with respect to the charge ZT for a wide
temperature range, above 300 K. These findings suggest the potential value of
graphene nanosystems as energy harvesting devices employing spin currents.

###Enhancing the Figure of Merit in Te-doped FeSb2 through nanostructuring|M. Pokharel,H. Z. Zhao,M. Koirala,Z. F. Ren,C. Opeil###

Enhancing the Figure of Merit in Te-doped FeSb2 through nanostructuring. We study the thermoelectric properties of Te-doped FeSb2 nanostructured
samples. Four samples of stoichiometry FeSb1.84Te0.16 were prepared by a hot
press method at temperatures of 200, 400, 500, and 600 oC. Te-doping enhances
the dimensionless figure of merit (ZT) on FeSb2 via two mechanisms. First, a
semiconductor to metal transition is induced, which enhances the value of the
power factor at low-temperatures. Second, the thermal conductivity, which was
already reduced in nanostructured FeSb2 samples, is further reduced by
increased point defect scattering through the n type substitution of Sb site by
Te atom. The combined effect results in a ZT = 0.022 at 100 K, an increase of
62% over the ZT value for the optimized Te-doped single crystal sample. Hall
coefficient and electrical resistivity measurements reveal a decreased mobility
and increased concentration of the carriers in the doped sample.

###Bilayer graphene nanoribbons junction with aligned holes exhibiting high ZT values|Shuo Deng,Lijie Li,Paul Rees###

Bilayer graphene nanoribbons junction with aligned holes exhibiting high ZT values. We investigate the thermoelectric performance of armchair graphene nanoribbon
(AGNR), bilayer GNRs junction (BGNRJ) and BGNRJ with holes (BGNRJ-H) by the
first principles calculation with non-equilibrium Green function. It is found
that the BGNRJ-H exhibits high ZT values of 9.65 and 5.55 at 300K. The reason
of these significantly larger ZT values than previously observed has been
calculated due to reduced thermal conductivity and enhanced electrical
conductivity. The low thermal conductance comes from the van der waals (vdW)
interaction between two graphene layers. The increased electrical conductivity
can be attributed to the coupling effect of aligned holes in both layers. It is
found from analysis results that the electron transmission of the BGNRJ-H is
much stronger than a normal BGNRJ, which gives rise to the higher electrical
conductance and outstanding ZT values.

###Thermoelectrics properties of two-dimensional materials with combination of linear and nonlinear band structures|Andri Darmawan,Edi Suprayoga,Ahmad R. T. Nugraha,Abdullah A. AlShaikhi###

Thermoelectrics properties of two-dimensional materials with combination of linear and nonlinear band structures. We investigate thermoelectric (TE) properties of two-dimensional materials
possessing two Dirac bands (a Dirac band) and a nonlinear band within the
three-(two-)band model using linearized Boltzmann transport theory and
relaxation time approximation. In the three-band model, we find that
combinations of Dirac bands with a heavy nonlinear band, either a parabolic or
a pudding-mold band, does not give much difference in their TE performance. The
apparent difference only occurs in the position of the nonlinear band that
leads to the maximum figure of merit ($ZT$). The optimum $ZT$ of the three-band
model consisting of a nonlinear band is found when the nonlinear band
intersects the Dirac bands near the Fermi level. By removing the linear
conduction band, or, in other words, transforming the three-band model to the
two-band model, we find better TE performance in the two-band model than in the
three-band model, i.e., in terms of higher $ZT$ values

###Exploring the possibility of enhancing the figure-of-merit ( $>$ 2) of Na$_{0.74}$CoO$_{2}$: A combined experimental and theoretical study|Shamim Sk,Jayashree Pati,R. S. Dhaka,Sudhir K. Pandey###

Exploring the possibility of enhancing the figure-of-merit ( $>$ 2) of Na$_{0.74}$CoO$_{2}$: A combined experimental and theoretical study. Search of new thermoelectric (TE) materials with high
\textit{figure-of-merit} (ZT) is always inspired the researcher in TE field.
Here, we present a combined experimental and theoretical study of TE properties
of Na$_{0.74}$CoO$_{2}$ compound in high-temperature region. The experimental
Seebeck coefficient (S) is found to vary from 64 to 118 $\mu$V/K in the
temperature range $300-620$ K. The positive values of S are indicating the
dominating p-type behaviour of the compound. The observed value of thermal
conductivity ($\kappa$) is $\sim$ 2.2 W/m-K at 300 K. In the temperature region
$300-430$ K, the value of $\kappa$ increases up to $\sim$ 2.6 W/m-K and then
decreases slowly till 620 K with the corresponding value of $\sim$ 2.4 W/m-K.
We have also carried out the theoretical calculations and the best matching
between experimental and calculated values of transport properties are observed
in spin-polarized calculation within DFT+\textit{U} by chosen \textit{U} = 4
eV. The maximum calculated value of ZT is found to be $\sim$ 0.67 at 1200 K for
p-type conduction. Our computational study suggests that the possibility of
n-type behaviour of the compound which can lead to a large value of ZT at
higher temperature region. Electron doping of $\sim$ 5.1$\times$10$^{20}$
cm$^{-3}$ is expected to give rise the high ZT value of $\sim$ 2.7 at 1200 K.
Using these temperature-dependent ZT values, we have calculated the maximum
possible values of efficiency ($\eta$) of thermoelectric generator (TEG) made
by p and n-type Na$_{0.74}$CoO$_{2}$. The present study suggests that one can
get the efficiency of a TE cell as high as $\sim$ 11$\%$ when the cold and hot
end temperature are fixed at 300 K and 1200 K, respectively. Such high values
of ZT and efficiency suggest that Na$_{0.74}$CoO$_{2}$ can be used as a
potential candidate for high-temperature TE applications.

###First-principles quantum transport modeling of thermoelectricity in single-molecule nanojunctions with graphene nanoribbon electrodes|Branislav K. Nikolic,Kamal K. Saha,Troels Markussen,Kristian S. Thygesen###

First-principles quantum transport modeling of thermoelectricity in single-molecule nanojunctions with graphene nanoribbon electrodes. We overview nonequilibrium Green function combined with density functional
theory (NEGF-DFT) modeling of independent electron and phonon transport in
nanojunctions with applications focused on a new class of thermoelectric
devices where a single molecule is attached to two metallic zigzag graphene
nanoribbons (ZGNRs) via highly transparent contacts. Such contacts make
possible injection of evanescent wavefunctions from ZGNRs, so that their
overlap within the molecular region generates a peak in the electronic
transmission. Additionally, the spatial symmetry properties of the transverse
propagating states in the ZGNR electrodes suppress hole-like contributions to
the thermopower. Thus optimized thermopower, together with diminished phonon
conductance through a ZGNR/molecule/ZGNR inhomogeneous structure, yields the
thermoelectric figure of merit ZT~0.5 at room temperature and 0.5<ZT<2.5 below
liquid nitrogen temperature. The reliance on evanescent mode transport and
symmetry of propagating states in the electrodes makes the
electronic-transport-determined power factor in this class of devices largely
insensitive to the type of sufficiently short conjugated organic molecule,
which we demonstrate by showing that both 18-annulene and C10 molecule
sandwiched by the two ZGNR electrodes yield similar thermopower. Thus, one can
search for molecules that will further reduce the phonon thermal conductance
(in the denominator of ZT) while keeping the electronic power factor (in the
nominator of ZT) optimized. We also show how often employed Brenner empirical
interatomic potential for hydrocarbon systems fails to describe phonon
transport in our single-molecule nanojunctions when contrasted with
first-principles results obtained via NEGF-DFT methodology.

###Spin-tunable thermoelectric performance in monolayer chromium pnictides|Melania S. Muntini,Edi Suprayoga,Sasfan A. Wella,Iim Fatimah,Lila Yuwana,Tosawat Seetawan,Adam B. Cahaya,Ahmad R. T. Nugraha,Eddwi H. Hasdeo###

Spin-tunable thermoelectric performance in monolayer chromium pnictides. Historically, finding two-dimensional (2D) magnets is well known to be a
difficult task due to instability against thermal spin fluctuations. Metals are
also normally considered poor thermoelectric (TE) materials. Combining
intrinsic magnetism in two dimensions with conducting properties, one may
expect to get the worst for thermoelectrics. However, we will show this is not
always the case. Here, we investigate spin-dependent TE properties of monolayer
chromium pnictides (CrX, where X = P, As, Sb, and Bi) using first-principles
calculations of electrons and phonons, along with Boltzmann transport formalism
under energy-dependent relaxation time approximation. All the CrX monolayers
are dynamically stable and they also exhibit half metallicity with
ferromagnetic ordering. Using the spin-valve setup with antiparallel spin
configuration, the half metallicity and ferromagnetism in monolayer CrX enable
manipulation of spin degrees of freedom to tune the TE figure of merit (ZT). At
optimized chemical potential and operating temperature of 500 K, the maximum ZT
values (= 0.22, 0.12, and 0.09) with the antiparallel spin-valve setup in CrAs,
CrSb, and CrBi improve up to almost twice the original values (ZT = 0.12, 0.08,
and 0.05) without the spin-valve configuration. Only in CrP, which is the
lightest species and less spin-polarized among CrX, the maximum ZT (= 0.34)
without the spin-valve configuration is larger than that (= 0.19) with the
spin-valve one. We also find that, at 500 K, all the CrX monolayers possess
exceptional TE power factors of about 0.02-0.08 W/m.K2, which could be one of
the best values among 2D conductors.

###Electron- and phonon transport in silicon nanowires: an atomistic approach to thermoelectric properties|Troels Markussen,Antti-Pekka Jauho,Mads Brandbyge###

Electron- and phonon transport in silicon nanowires: an atomistic approach to thermoelectric properties. We compute both electron- and phonon transmissions in thin disordered silicon
nanowires. Our atomistic approach is based on tight-binding and empirical
potential descriptions of the electronic and phononic systems, respectively.
Surface disorder is modeled by including surface silicon vacancies. It is shown
that the average phonon- and electron transmissions through long SiNWs
containing many vacancies can be accurately estimated from the scattering
properties of the isolated vacancies using a recently proposed averaging method
[Phys. Rev. Lett. 99, 076803 (2007)]. We apply this averaging method to surface
disordered SiNWs in the diameter range 1-3 nm to compute the thermoelectric
figure of merit, ZT. It is found that the phonon transmission is affected more
by the vacancies than the electronic transmission leading to an increased
thermoelectric performance of disordered wires, in qualitative agreement with
recent experiments. The largest ZT>3 is found in strongly disordered <111>
oriented wires with a diameter of 2 nm.

###Nonlinear thermoelectricity in point-contacts at pinch-off: a catastrophe aids cooling|Robert S. Whitney###

Nonlinear thermoelectricity in point-contacts at pinch-off: a catastrophe aids cooling. We consider refrigeration and heat engine circuits based on the nonlinear
thermoelectric response of point-contacts at pinch-off, allowing for
electrostatic interaction effects. We show that a refrigerator can cool to much
lower temperatures than predicted by the thermoelectric figure-of-merit ZT
(which is based on linear-response arguments). The lowest achievable
temperature has a discontinuity, called a fold catastrophe in mathematics, at a
critical driving current I=I_c. For I >I_c one can in principle cool to
absolute zero, when for I<I_c the lowest temperature is about half the ambient
temperature. Heat back-flow due to phonons and photons stop cooling at a
temperature above absolute zero, and above a certain threshold turns the
discontinuity into a sharp cusp. We also give a heuristic condition for when an
arbitrary system's nonlinear response means that its ZT ceases to indicate
(even qualitatively) the lowest temperature to which the system can
refrigerate.

###Doping effect on thermoelectric properties of MoS$_2$|Huaihong Guo,Teng Yang,Peng Tao,Zhidong Zhang###

Doping effect on thermoelectric properties of MoS$_2$. We systematically study thermoelectric properties of layered MoS$_2$ by
doping, based on Boltzmann transport theory and first-principles calculations.
We obtain optimal doping region (around 10$^{19}$ cm$^{-3}$) by looking closely
to the temperature and doping level dependent thermopower, electrical
conductivity, power factor (PF) and ultimately figure of merit (ZT) coefficient
along in-plane and cross-plane directions. MoS$_2$ has a vanishingly small
anisotropy of thermopower but a big anisotropy of electrical conductivity and
electronic thermal conductivity in optimal doping region. $\kappa_e$ is
comparable to $\kappa_l$ in the plane while $\kappa_l$ dominates over
$\kappa_e$ across the plane. ZT can reach as high as 0.3 at around 700 K.
In-plane direction is demonstrated to be more preferable for thermoelectric
applications of MoS$_2$ by doping.

###Thermoelectric effects of quantum dot arrays embedded in nanowires|Yen-Chun Tseng,David M. -T. Kuo,Yia-Chung Chang,Chia-Wei Tsai###

Thermoelectric effects of quantum dot arrays embedded in nanowires. The thermoelectric properties of quantum dot arrays (QDAs) embedded in
nanowires connected to electrodes are studied theoretically in the Coulomb
blockade regime. A Hurbbard-Anderson model is used to simulate the electronic
contribution to thermoelectric proper- ties of a QDA junction system. The
electrical conductance, Seebeck coefficient, and electron thermal conductance
are calculated by both the Keldysh Green function method and the mean-field
approach. The phonon thermal conductivities are calculated by using the
equation of phonon radiative transfer method. In the Coulomb blockade regime
the electron thermal conductance is much smaller than the phonon thermal
conductance. Therefore, the optimal figure of merit (ZT) can be enhanced by
increasing thermal power and decreasing phonon thermal conductance
simultaneously. We found that it is possible to obtain ZT value of InGaAs/GaAs
QDAs embedded in nanowires larger than one at room temperature.

###High thermoelectric performance of distorted Bismuth (110) layer|L. Cheng,H. J. Liu,J. Zhang,J. Wei,J. H. Liang,P. H. Jiang,D. D. Fan,L. Sun,J. Shi###

High thermoelectric performance of distorted Bismuth (110) layer. The thermoelectric properties of distorted bismuth (110) layer are
investigated using first-principles calculations combined with the Boltzmann
transport equation for both electrons and phonons. To accurately predict the
electronic and transport properties, the quasiparticle corrections with the GW
approximation of many-body effects have been explicitly included. It is found
that a maximum ZT value of 6.4 can be achieved for n-type system, which is
essentially stemmed from the weak scattering of electrons. Moreover, we
demonstrate that the distorted Bi layer remains high ZT values at relatively
broad regions of both temperature and carrier concentration. Our theoretical
work emphasizes that the deformation potential constant characterizing the
electron-phonon scattering strength is an important paradigm for searching high
thermoelectric performance materials.

###Thermoelectric radiation detector based on a superconductor-ferromagnet junction: calorimetric regime|Subrata Chakraborty,Tero T. Heikkilä###

Thermoelectric radiation detector based on a superconductor-ferromagnet junction: calorimetric regime. We study the use of a thermoelectric junction as a thermal radiation detector
in the calorimetric regime, where single radiation bursts can be separated in
time domain. We focus especially on the case of a large thermoelectric figure
of merit $ZT$ affecting significantly for example the relevant thermal time
scales. This work is motivated by the use of hybrid superconductor/ferromagnet
systems in creating an unprecedentedly high low-temperature $ZT$ even exceeding
unity. Besides constructing a very general noise model which takes into account
cross correlations between charge and heat noise, we show how the detector
signal can be efficiently multiplexed by the use of resonant LC circuits giving
a fingerprint to each pixel. We show that for realistic detectors operating at
temperatures around 100 to 200 mK, the energy resolution can be as low as 1
meV. This allows for a broadband single-photon resolution at photon frequencies
of the order or below 1 THz.

###Thermoelectric Properties of Intermetallic Semiconducting RuIn3 and Metallic IrIn3|N. Haldolaarachchige,W. A. Phelan,Y. M. Xiong,R. Jin,J. Y. Chan,S. Stadler,D. P. Young###

Thermoelectric Properties of Intermetallic Semiconducting RuIn3 and Metallic IrIn3. Low temperature (<400 K) thermoelectric properties of semiconducting RuIn3
and metallic IrIn3 are reported. RuIn3 is a narrow band gap semiconductor with
a large n-type Seebeck coefficient at room temperature (S(290K)~400 {\mu}V/K),
but the thermoelectric Figure of merit (ZT(290K) = 0.007) is small because of
high electrical resistivity and thermal conductivity ({\kappa}(290 K) ~ 2.0 W/m
K). IrIn3 is a metal with low thermopower at room temperature (S(290K)~20
{\mu}V/K) . Iridium substitution on the ruthenium site has a dramatic effect on
transport properties, which leads to a large improvement in the power factor
and corresponding Figure of merit (ZT(380 K) = 0.053), improving the efficiency
of the material by an over of magnitude.

###Large thermoelectric power factors in black phosphorus and phosphorene|H. Y. Lv,W. J. Lu,D. F. Shao,Y. P. Sun###

Large thermoelectric power factors in black phosphorus and phosphorene. The electronic properties of the layered black phosphorus (black-P) and its
monolayer counterpart phosphorene are investigated by using the
first-principles calculations based on the density functional theory (DFT). The
room-temperature electronic transport coefficients are evaluated within the
semi-classical Boltzmann theory. The electrical conductivity exhibits
anisotropic behavior while the Seebeck coefficient is almost isotropic. At the
optimal doping level and room temperature, bulk black-P and phosphorene are
found to have large thermoelectric power factors of 118.4 and 138.9
{\mu}Wcm-1K-2, respectively. The maximum dimensionless figure of merit (ZT
value) of 0.22 can be achieved in bulk black-P by appropriate n-type doping,
primarily limited by the reducible lattice thermal conductivity. For the
phosphorene, the ZT value can reach 0.30 conservatively estimated by using the
bulk lattice thermal conductivity. Our results suggest that both bulk black-P
and phosphorene are potentially promising thermoelectric materials.

###Enhancement of thermoelectric performance in Graphene/BN heterostructures|Van-Truong Tran,Jérôme Saint Martin,Philippe Dollfus###

Enhancement of thermoelectric performance in Graphene/BN heterostructures. The thermoelectric properties of in plane heterostructures made of Graphene
and hexagonal Boron Nitride (BN) have been investigated by means of atomistic
simulation. The heterostructures consist in armchair graphene nanoribbons to
the sides of which BN flakes are periodically attached. This arrangement
generates a strong mismatch of phonon modes between the different sections of
the ribbons, which leads to a very small phonon conductance, while the electron
transmission is weakly affected. In combination with the large Seebeck
coefficient resulting from the BN-induced bandgap opening or broadening, it is
shown that large thermoelectric figure of merit ZT > 0.8 can be reached in
perfect structures at relatively low Fermi energy, depending on the graphene
nanoribbon width. The high value ZT = 1.48 may even be achieved by introducing
appropriately vacancies in the channel, as a consequence of further degradation
of the phonon conductance.

###Strain-induced enhancement of thermoelectric performance in a ZrS2 monolayer|H. Y. Lv,W. J. Lu,D. F. Shao,H. Y. Lu,Y. P. Sun###

Strain-induced enhancement of thermoelectric performance in a ZrS2 monolayer. The increase of a thermoelectric material's figure of merit (ZT value) is
limited by the interplay of the transport coefficients. Here we report the
greatly enhanced thermoelectric performance of a ZrS2 monolayer by the biaxial
tensile strain, due to the simultaneous increase of the Seebeck coefficient and
decrease of the thermal conductivity. Based on the first-principles
calculations combined with the Boltzmann transport theory, we predict the band
gap of the ZrS2 monolayer can be effectively engineered by the strain and the
Seebeck coefficient is significantly increased. The thermal conductivity is
reduced by the applied tensile strain due to the phonon softening. At the
strain of 6%, the maximal ZT value of 2.4 is obtained for the p-type doped ZrS2
monolayer at 300 K, which is 4.3 times larger than that of the unstrained
system.

###Optimal thermoelectric figure of merit of a molecular junction|Padraig Murphy,Subroto Mukerjee,Joel Moore###

Optimal thermoelectric figure of merit of a molecular junction. We show that a molecular junction can give large values of the thermoelectric
figure of merit $ZT$, and so could be used as a solid state energy conversion
device that operates close to the Carnot efficiency. The mechanism is similar
to the Mahan-Sofo model for bulk thermoelectrics -- the Lorenz number goes to
zero violating the Wiedemann-Franz law while the thermopower remains non-zero.
The molecular state through which charge is transported must be weakly coupled
to the leads, and the energy level of the state must be of order $k_B T$ away
from the Fermi energy of the leads. In practice, the figure of merit is limited
by the phonon thermal conductance; we show that the largest possible
$ZT\sim(\tilde{G}_{th}^{ph})^{-1/2}$, where $\tilde{G}_{th}^{ph}$ is the phonon
thermal conductance divided by the thermal conductance quantum.

###Unexpected band gap increase in the Fe2VAl Heusler compound|A. Berche,M. Talla Noutack,M. -L. Doublet,P. Jund###

Unexpected band gap increase in the Fe2VAl Heusler compound. Knowing the electronic structure of a material is essential in energy
applications to rationalize its performance and propose alternatives. Materials
for thermoelectric applications are generally small-gap semiconductors and
should have a high figure of merit ZT. Even if the Fe2VAl Heusler compound has
a decent ZT, its conductive nature (semi-metal or semiconductor) is not yet
clarified especially at low temperature. In this paper, we focus our DFT
calculations on the effect of temperature on the bandgap of Fe2VAl. In contrast
to what is usually observed, we show that both the temperature increase and the
formation of thermally-activated Al/V inversion defects (observed
experimentally), open the bandgap. Such an unusual behavior is the key for
reconciling all bandgap measurements performed on the Fe2VAl compound using a
standard GGA functional and could be an efficient way for improving the
thermoelectric properties of this family of materials.

###Electronic and Thermoelectric Properties of Few-Layer Transition Metal Dichalcogenides|Darshana Wickramaratne,Ferdows Zahid,Roger K. Lake###

Electronic and Thermoelectric Properties of Few-Layer Transition Metal Dichalcogenides. The electronic and thermoelectric properties of one to four monolayers of
MoS$_{2}$, MoSe$_{2}$, WS$_{2}$, and WSe$_{2}$ are calculated. For few layer
thicknesses,the near degeneracies of the conduction band $K$ and $\Sigma$
valleys and the valence band $\Gamma$ and $K$ valleys enhance the n-type and
p-type thermoelectric performance. The interlayer hybridization and energy
level splitting determine how the number of modes within $k_BT$ of a valley
minimum changes with layer thickness. In all cases, the maximum ZT coincides
with the greatest near-degeneracy within $k_BT$ of the band edge that results
in the sharpest turn-on of the density of modes. The thickness at which this
maximum occurs is, in general, not a monolayer. The transition from few layers
to bulk is discussed. Effective masses, energy gaps, power-factors, and ZT
values are tabulated for all materials and layer thicknesses.

###Does Topology Enhance Thermoelectric Efficiency? A Case Study in Bismuthene|Muhammad Gaffar,Sasfan Arman Wella,Eddwi Hesky Hasdeo###

Does Topology Enhance Thermoelectric Efficiency? A Case Study in Bismuthene. Two-dimensional (2D) bismuth (Bi) layer, known as bismuthene, exhibits $Z2$
topological bulk states due to large spin-orbit coupling that inverts the
bands. Using the tight-binding method, we calculate the band structure of
buckled bismuthene to understand its topological and trivial phases. We
determine the thermoelectric properties for some considered phases,
incorporating the edge states contribution, by using the linearized Boltzmann
transport equation (BTE) with a constant relaxation time approximation. It is
shown that the thermoelectric figure of merit, $ZT$, actually drops in undoped
topological bismuthene due to the edge effects. Surprisingly, the topological
edge states enhance $ZT$ at large doping with the Fermi energy near the bottom
of bulk bands when bismuthene is nearly metallic.

###Electrical and Thermal transport studies of Sr and Mn co-substituted NdCoO3|Ashutosh Kumar###

Electrical and Thermal transport studies of Sr and Mn co-substituted NdCoO3. Oxide thermoelectrics are exciting due to their chemical and thermal
stability at high temperatures. However, the efficacy of these materials are
limited by poor figure of merit (zT). In this study, the role of Sr and Mn
co-substitution on the thermoelectric properties of NdCoO3
(Nd_{1-x}Sr_xCo_{1-y}Mn_yO_3; 0.00 \leq x \leq 0.10; 0.00 \leq y \leq 0.10) is
investigated. The Seebeck coefficient decreases with single Sr substitution at
Nd site; however, the Sr and Mn co-substitution enhances the Seebeck
coefficient compared to single Sr substitution and is attributed to the
localization effect. Sr substitution at La site creates hole in the system and
results in enhanced electrical conductivity ({\sigma}); however, {\sigma}
reduces with Mn substitution at Co site in NdCoO_3. A reduced thermal
conductivity for the co-substituted samples is observed and attributed to
decrease in phonon thermal conductivity. Simultaneous optimization of TE
parameters results in improved zT \sim 0.038 for
Nd_{0.95}Sr_{0.05}Co_{0.95}Mn_{0.05}O_3 at 540 K.

###Electronic and phonon contributions to the Thermoelectric properties of newly discovered half-Heusler alloys XHfPb (X= Ni, Pd, and Pt)|Paul O. Adebambo,Gboyega A. Adebayo,Roberto Guerra,Davide Ceresoli###

Electronic and phonon contributions to the Thermoelectric properties of newly discovered half-Heusler alloys XHfPb (X= Ni, Pd, and Pt). In this work we calculate the thermoelectric figure of merit of XHfPb (X= Ni,
Pd, and Pt) by computing the both the power factor and the lattice thermal
conductivity by first principles. We make reasonable approximations: we use the
Constant Relaxation Time Approximation (CRTA) to compute the electron transport
contribution and the modified Debye-Callaway model to calculate the thermal
lattice conductivity. We also report the dielectric properties of these
semiconductors and the mode Gr\"uneisen parameters. Not surprisingly we find
that the average Gr\"uneisen coefficient correlates with the tehrmal
conductivity. Next, we consider a realistic relaxation time $\tau$ and carrier
concentration $n$ from experimental data on ZrHfPb and obtain the figure of
merit $ZT$ as a function of temperature. Our main finding is that despite the
Pt is isoelectronic with Ni and Pd, the $ZT$ of PtHfPb is larger and behaves
differently from the other two materials, suggesting that PtHfPb is better
suited for high temperature thermoelectric generators.

###Monolayer enhanced thermoelectric properties compared with bulk for BiTeBr|San-Dong Guo,Hui-Chao Li###

Monolayer enhanced thermoelectric properties compared with bulk for BiTeBr. It is believed that nanostructuring is an effective way to achieve excellent
thermoelectric performance. In the work, by combining the first-principles
calculations and semiclassical Boltzmann transport theory, we investigate the
thermoelectric properties of bulk and monolayer BiTeBr including both the
electron and phonon transports. The generalized gradient approximation (GGA)
plus spin-orbit coupling (SOC) is employed for the electron part, and GGA for
the phonon part. It is found that SOC has important effects on electronic
transport coefficients because of SOC-induced obvious influences on energy band
structures. In p-type doping, monolayer has larger Seebeck coefficient than
bulk in wide doping range, which is beneficial to excellent thermoelectric
performance. The calculated average lattice thermal conductivity of bulk is
1.71 $\mathrm{W m^{-1} K^{-1}}$ at room temperature, which is close to
experimental value 1.3 $\mathrm{W m^{-1} K^{-1}}$. Calculated results show that
monolayer has better $ZT_e$ and lower lattice thermal conductivity than bulk,
which suggests that monolayer has better thermoelectric performance than bulk.
The lower lattice thermal conductivity in monolayer than bulk is due to shorter
phonon lifetimes. By comparing the experimental electrical conductivity of bulk
with calculated value, the scattering time is determined for 3.3 $\times$
$10^{-14}$ s. Based on electron and phonon transport coefficients, the
thermoelectric figure of merit $ZT$ of bulk and monolayer are calculated. It is
found that monolayer has higher peak $ZT$ than bulk, and the peak $ZT$ of
monolayer can be as high as 0.55 in n-type doping and 0.75 in p-type doping at
room temperature. These results imply that monolayer BiTeBr may be a potential
two-dimensional (2D) thermoelectric material, which can stimulate further
experimental works to synthesize monolayer BiTeBr.

###A microscopic mechanism for increasing thermoelectric efficiency|Keiji Saito,Giuliano Benenti,Giulio Casati###

A microscopic mechanism for increasing thermoelectric efficiency. We study the coupled particle and energy transport in a prototype model of
interacting one-dimensional system: the disordered hard-point gas, for which
numerical data suggest that the thermoelectric figure of merit ZT diverges with
the system size. This result is explained in terms of a microscopic mechanism,
namely the local equilibrium is characterized by the emergence of a broad
stationary "modified Maxwell-Boltzmann velocity distribution", of width much
larger than the mean velocity of the particle flow.

###Enhancement of thermoelectric properties by Se substitution in layered bismuth-chalcogenide LaOBiS2-xSex|Yoshikazu Mizuguchi,Atsushi Omachi,Yosuke Goto,Yoichi Kamihara,Masanori Matoba,Takafumi Hiroi,Joe Kajitani,Osuke Miura###

Enhancement of thermoelectric properties by Se substitution in layered bismuth-chalcogenide LaOBiS2-xSex. We have investigated the thermoelectric properties of the novel layered
bismuth chalcogenides LaOBiS2-xSex. The partial substitution of S by Se
produced the enhancement of electrical conductivity (metallic characteristics)
in LaOBiS2-xSex. The power factor largely increased with increasing Se
concentration. The highest power factor was 4.5 uW/cmK2 at around 470 deg. C
for LaOBiS1.2Se0.8. The obtained dimensionless figure-of-merit (ZT) was 0.17 at
around 470 deg. C in LaOBiS1.2Se0.8.

###LaPtSb: a half-Heusler compound with high thermoelectric performance|Q. Y. Xue,H. J. Liu,D. D. Fan,L. Cheng,B. Y. Zhao,J. Shi###

LaPtSb: a half-Heusler compound with high thermoelectric performance. The electronic and transport properties of the half-Heusler compound LaPtSb
are investigated by performing first-principles calculations combined with
semi-classical Boltzmann theory and deformation potential theory. Compared with
many typical half-Heusler compounds, the LaPtSb exhibits obviously larger power
factor at room temperature, especially for the n-type system. Together with the
very low lattice thermal conductivity, the thermoelectric figure of merit (ZT)
of LaPtSb can be optimized to a record high value of 2.2 by fine tuning the
carrier concentration.

###Thermoelectric Figure of Merit of Strongly Correlated Superlattice Semiconductors|Wenjin Mao,Kevin S. Bedell###

Thermoelectric Figure of Merit of Strongly Correlated Superlattice Semiconductors. We solved the Anderson Lattice Hamiltonian to get the energy bands of a
strongly correlated semiconductor by using slave boson mean field theory. The
transport properties were calculated in the relaxation-time approximation,and
the thermoelectric figure of merit was obtained for the strongly correlated
semiconductor and its superlattice structures. We found that at room
temperature $ZT$ can reach nearly 2 for the quantum wire lattice structure.We
believe that it is possible to find high values of thermoelectric figure of
merit from strongly correlated semiconductor superlattice systems.

###Thermal electric effects and heat generation in polypyrrole coated PET fabrics|J. Avloni,L. Florio,A. R. Henn,A. Sparavigna###

Thermal electric effects and heat generation in polypyrrole coated PET fabrics. Polypyrrole chemically synthesized on PET gives rise to textiles with a high
electric conductivity, suitable for several applications from antistatics to
electromagnetic interference shielding devices. Here, we discuss investigations
on thermal electric performances of the polypyrrole coated PET in a wide range
of temperatures above room temperature. The Seebeck coefficient turns out to be
comparable with that of metal thermocouple materials. Since polypyrrole shows
extremely low thermal diffusivities regardless of the electrical conductivity,
the low thermal conductivity gives significant advantage to the thermoelectric
figure-of-merit ZT, comparable with that of some traditional inorganic
thermoelectric materials. The heat generation is also investigated for possible
heating textile devices.

###Enhanced thermoelectric figure of merit in edge disordered zigzag graphene nanoribbons|H. Sevincli,G. Cuniberti###

Enhanced thermoelectric figure of merit in edge disordered zigzag graphene nanoribbons. We investigate electron and phonon transport through edge disordered zigzag
graphene nanoribbons based on the same methodological tool of nonequilibrium
Green functions. We show that edge disorder dramatically reduces phonon thermal
transport while being only weakly detrimental to electronic conduction. The
behavior of the electronic and phononic elastic mean free paths points to the
possibility of realizing an electron-crystal coexisting with a phonon-glass.
The calculated thermoelectric figure of merit (ZT) values qualify zigzag
graphene nanoribbons as a very promising material for thermoelectric
applications.

###New type of thermoelectric conversion of energy by semiconducting liquid anisotropic media|Sergey I. Trashkeev,Alexey N. Kudryavtsev###

New type of thermoelectric conversion of energy by semiconducting liquid anisotropic media. The paper describes preliminary investigations of a new effect in conducting
anisotropic liquids, which leads to thermoelectric conversion of energy.
Nematic liquid crystals with semiconducting dopes are used. A thermoelectric
figure of merit ZT = 0.2 is obtained in experiments. The effect can be
explained by assuming that the thermocurrent in semiconducting nematics, in
contrast to the Seebeck effect, is a nonlinear function of the temperature
gradient and of the temperature itself. Though the discovered effect has to be
further investigated, the data obtained suggest that it can be effectively used
in alternative energy engineering.

###Spin Seebeck Power Conversion|Adam B. Cahaya,Oleg A. Tretiakov,G. E. W. Bauer###

Spin Seebeck Power Conversion. Spin caloritronics is the science and technology to control spin, charge, and
heat currents in magnetic nanostructures. The spin degree of freedom provides
new strategies for thermolelectric power generation that have not yet been
fully explored. After an elementary introduction into conventional
thermoelectrics and spintronics, we give a brief review of the physics of spin
caloritronics. We discuss spin-dependent thermoelectrics based on the the
two-current model in metallic magnets as well as the spin Seebeck and Peltier
effects that are based on spin wave excitations in ferromagnets. We derive
expressions for the efficiency and figure of merit ZT of several spin
caloritronic devices.

###High thermoelectric performance can be achieved in black phosphorus|J. Zhang,H. J. Liu,L. Cheng,J. Wei,J. H. Liang,D. D. Fan,P. H. Jiang,L. Sun,J. Shi###

High thermoelectric performance can be achieved in black phosphorus. Few-layer black phosphorus has recently emerged as a promising candidate for
novel electronic and optoelectronic device. Here we demonstrate by
first-principles calculations and Boltzmann theory that, black phosphorus could
also have potential thermoelectric applications and a fair ZT value of 1.1 can
be achieved at elevated temperature. Moreover, such value can be further
increased to 5.4 by substituting P atom with Sb atom, giving nominal formula of
P0.75Sb0.25. Our theoretical work suggests that high thermoelectric performance
can be achieved without using complicated crystal structure or seeking for
low-dimensional systems.

###Thermoelectric characterization of fine-grained Ti5O9 Magneli phase ceramics|Sudeep Jung Pandey,Giriraj Joshi,Shidong Wang,Stefano Curtarolo,Romain Gaume###

Thermoelectric characterization of fine-grained Ti5O9 Magneli phase ceramics. Magneli phase Ti5O9 ceramics with 200-nm grain-size were fabricated by
hot-pressing nanopowders of titanium and anatase TiO2 at 1223 K. The
thermoelectric properties of these ceramics were investigated from room
temperature to 1076 K. We show that the experimental variation of the
electrical conductivity with temperature follows a small-polaron model and that
the Seebeck coefficient can be explained by a temperature-dependent
Heikes-Chaikin-Beni model. The thermoelectric figure-of-merit ZT of this
nanoceramic material reaches 0.3 at 1076 K.

###First-principles study of the thermoelectric properties of Zintl compound KSnSb|S. Huang,H. J. Liu,D. D. Fan,P. H. Jiang,J. H. Liang,G. H. Cao,J. Shi###

First-principles study of the thermoelectric properties of Zintl compound KSnSb. The unique structure of Zintl phase makes it an ideal system to realize the
concept of phonon-glass and electron-crystal in the thermoelectric community.
In this work, by combining first-principles calculations and Boltzmann
transport theory for both electrons and phonons, we demonstrate that the ZT
value of Zintl compound KSnSb can reach ~2.6 at 800 K. Such extraordinary
thermoelectric performance originates from the large Seebeck coefficient due to
multi-valley band structures and particularly very small lattice thermal
conductivity caused by mixed-bond characteristics.

###How bilayer excitons can greatly enhance thermoelectric efficiency|Kai Wu,Louk Rademaker,Jan Zaanen###

How bilayer excitons can greatly enhance thermoelectric efficiency. Currently, one of the major nanotechnological challenges is to design
thermoelectric devices that have a high figure of merit. To that end, we
propose to use bilayer excitons. Bilayer exciton systems are shown to have an
improved thermopower and an enhanced electric counterflow and thermal
conductivity, with respect to regular semiconductor-based thermoelectrics. Here
we present a roadmap towards experimental realization of a bilayer exciton
thermocouple. A bilayer exciton heterostructures of $p$- and $n$-doped
Bi$_2$Te$_3$ can have a figure of merit $zT \sim 60$. Another material
suggestion is to make a bilayer out of electron-doped SrTiO$_3$ and hole-doped
Ca$_3$Co$_4$O$_9$.

###An Analytic Study of the Wiedemann-Franz Law and the Thermoelectric Figure of Merit|Aakash Yadav,PC Deshmukh,Ken Roberts,NM Jisrawi,SR Valluri###

An Analytic Study of the Wiedemann-Franz Law and the Thermoelectric Figure of Merit. Advances in optimizing thermoelectric material efficiency have seen a
parallel activity in theoretical and computational advances. In the current
work, it is shown that the calculation of exact Fermi-Dirac integrals enables
the generalization of the Wiedemann-Franz law (WF) to optimize the
dimensionless thermoelectric figure of merit ZT. This is done by optimizing the
Seebeck coefficient, the electrical conductivity and the thermal conductivity.
In the calculation of the thermal conductivity, both electronic and phononic
contributions are included. The solutions provide insight into the relevant
parameter space including the physical significance of complex solutions and
their dependence on the scattering parameter r and the reduced chemical
potential.

###High thermoelectric performance of half-Heusler compound BiBaK with intrinsically low lattice thermal conductivity|S. H. Han,Z. Z. Zhou,C. Y. Sheng,J. H. Liu,L. Wang,H. M. Yuan,H. J. Liu###

High thermoelectric performance of half-Heusler compound BiBaK with intrinsically low lattice thermal conductivity. Half-Heusler compounds usually exhibit relatively higher lattice thermal
conductivity that is undesirable for thermoelectric applications. Here we
demonstrate by first-principles calculations and Boltzmann transport theory
that the BiBaK system is an exception, which has rather low thermal
conductivity as evidenced by very small phonon group velocity and relaxation
time. Detailed analysis indicates that the heavy Bi and Ba atoms form a
cage-like structure, inside which the light K atom rattles with larger atomic
displacement parameters. In combination with its good electronic transport
properties, the BiBaK shows a maximum n-type ZT value of 1.9 at 900 K, which
outperforms most half-Heusler thermoelectric materials.

###Sharp enhancement on thermoelectric figure-of-merit of post-transition metal chalcogenides (PTMCs) using heterostructures with Mexican-hat valence band|Marcel S Claro###

Sharp enhancement on thermoelectric figure-of-merit of post-transition metal chalcogenides (PTMCs) using heterostructures with Mexican-hat valence band. Post-transition metal chalcogenides (PTMCs) such as GaSe, GaS, InSe, and InS
have been proposed as promising thermoelectric materials due to low lattice
conductivity, originating from the atomically layered structure, high Seebeck
coefficient, and the anticipation that its figure-of-merit be improved when
thinned to few-layers as the band structure turns into Mexican-hat valence band
(MHVB). Here we show by ab initio calculations that the MHVB should be present
even on thick films of InSe/GaSe type-II heterostructures, and a 50%
enhancement on thermoelectric figure-of-merit zT at room-temperature is
expected when compared with bulk InSe.

###High thermoelectric figure of merit in p-type Half-Heuslers by intrinsic phase separation|Elisabeth Rausch,Siham Ouardi,Ulrich Burkhardt,Claudia Felser,Jana Marie Stahlhofen,Benjamin Balke###

High thermoelectric figure of merit in p-type Half-Heuslers by intrinsic phase separation. Improvements in the thermoelectric properties of Half-Heusler materials have
been achieved by means of a micrometer-scale phase separation that increases
the phonon scattering and reduces the lattice thermal conductivity. A detailed
study of the p-type Half-Heusler compounds Ti(1-x)Hf(x)CoSb0.85Sn0.15 using
high-resolution synchrotron powder X-ray diffraction and element mapping
electron microscopy evidences the outstanding thermoelectric properties of this
system. A combination of intrinsic phase separation and adjustment of the
carrier concentration via Sn substitution is used to realize a record
thermoelectric figure of merit for p-type Half-Heusler compounds of ZT around
1.15 at 710C in Ti0.25Hf0.75CoSb0.85Sn0.15. The phase separation approach can
form a significant alternative to nanostructuring processing time, energy
consumption and increasing the thermoelectric efficiency.

###Strain effects to optimize the thermoelectric properties of hole-doped La$_2$NiO$_{4+δ}$ via ab initio calculations|Victor Pardo,Antia S. Botana,Daniel Baldomir###

Strain effects to optimize the thermoelectric properties of hole-doped La$_2$NiO$_{4+δ}$ via ab initio calculations. Thermoelectric properties of the system La$_2$NiO$_{4+\delta}$ have been
recently discussed [Phys. Rev. B 86, 165114 (2012)] via ab initio calculations.
An optimum hole-doping value was obtained with reasonable thermopower and
thermoelectric figure of merit being calculated. Here, a large increase in the
thermoelectric performance through lattice strain and the corresponding atomic
relaxations is predicted. This increase would be experimentally attainable via
growth in thin films of the material on top of different substrates. A small
tensile strain would produce large thermoelectric figures of merit at high
temperatures, $zT$ $\sim$ 1 in the range of oxygen excess $\delta$ $\sim$ 0.05
- 0.10 and in-plane lattice parameter in the range 3.95 - 4.05 \AA. In that
relatively wide range of parameters, thermopower values close to 200 $\mu$V/K
are obtained. The best performance of this compound is expected to occur in the
high temperature limit.

###Stability, Tunneling Characteristics and Thermoelectric Properties of TeSe2 allotropes|Munish Sharma###

Stability, Tunneling Characteristics and Thermoelectric Properties of TeSe2 allotropes. The waste heat management becomes very important with increasing energy
demand and limited fossil resources. Here, we demonstrate thermoelectric
performance of allotropic TeSe2. Based on the first-principle calculations, we
confirm the energetic and kinetic stability of five TeSe2 allotropes. We
predict {\delta}-TeSe2 as a new direct band gap semiconductor having 1.60 eV
direct band gap. All the TeSe2 allotropes exhibit band gap in UV-Vis region.
The structural phases are clearly distinguished using simulated scanning tunnel
microscopy. The room temperature Seebeck coefficient is maximum of 4 V/K for
{\delta}-TeSe2. We show that room temperature thermoelectric figure of merit
(ZT) can reach up to 3.1 with p-type doping in {\delta}-TeSe2. Moreover,
temperature and chemical potential tuning extends the thermoelectric
performance of TeSe2 allotropes. We strongly believe that our study is
compelling from an experimental perspective and holds a key towards fabrication
of thermoelectric devices based on TeSe2.

###Thermoelectric phenomenon in hollow blocks|M. Wehbe,J. Dgheim,E. Sassine###

Thermoelectric phenomenon in hollow blocks. The work presented in this article describes thermoelectric effect in hollow
blocks for heat waste harvesting purposes. The study consists of developing a
numerical model formed by a heat transfer equation coupled to thermoelectric
effects equations to study thermoelectric generators(TEG) incorporated inside
Lebanese hollow blocks through two simulations using finite difference scheme
and using finite element scheme. Results showed a voltage of 5.85mV produced
from a single 8.6 x 0.4 x 0.4 cm3 thermoelectric leg made of Bismuth Antimony
Telluride for {\Delta}T=30K. A design with 3 TEGs incorporated inside a hollow
block was tested and validated numerically using both methods, the main results
obtained for {\Delta}T=30K, showed a voltage {\Delta}V=0.72V, a current I=0.06
A and a figure of merit ZT=0.55. The design was then optimized for economic
purposes.

###Enhanced Thermoelectric ZT in the Tails of the Fermi Distribution via Electron Filtering by Nanoinclusions -- Model Electron Transport in Nanocomposites|S. Aria Hosseini,Devin Coleman,Sabah Bux,P. Alex Greaney,Lorenzo Mangolini###

Enhanced Thermoelectric ZT in the Tails of the Fermi Distribution via Electron Filtering by Nanoinclusions -- Model Electron Transport in Nanocomposites. Silicon carbide nanoparticles with diameters around 8 nm and with narrow size
distribution have been finely mixed with doped silicon nanopowders and sintered
into bulk samples to investigate the influence of nanoinclusions on electrical
and thermal transport properties. We have compared the thermoelectric
properties of samples ranging from 0-5% volume fraction of silicon carbide. The
silicon carbide nanoinclusions lead to a significant improvement in the
thermoelectric figure of merit, ZT, largely due to an enhancement of the
Seebeck coefficient. A semiclassical Boltzmann transport equation is used to
model the electrical transport properties of the Seebeck coefficient and
electrical conductivity. The theoretical analysis confirms that the
enhancements in the thermoelectric properties are consistent with the energy
selective scattering of electrons induced by the offset between the silicon
Fermi level and the carbide conduction band edge. This study proves that
careful engineering of the energy-dependent electron scattering rate can
provide a route towards relaxing long-standing constraints in the design of
thermoelectric materials.

###Outstanding thermoelectric performance predicted for out-of-plane p-doped GeSe|Anderson S. Chaves,Daniel T. Larson,Efthimios Kaxiras,Alex Antonelli###

Outstanding thermoelectric performance predicted for out-of-plane p-doped GeSe. The record-breaking thermoelectric performance of tin selenide (SnSe) has
motivated the investigation of analogue compounds with the same structure. A
promising candidate that emerged recently is germanium selenide (GeSe). Here,
using extensive first-principles calculations of the hole-phonon and
hole-impurity scattering, we investigate the thermoelectric transport
properties of the orthorhombic phase of p-doped GeSe. We predict outstanding
thermoelectric performance for GeSe over a broad range of temperatures due to
its high Seebeck coefficients, extremely low Lorenz numbers, ultralow total
thermal conductivity, and relatively large band gap. In particular, the
out-of-plane direction in GeSe presents equivalent or even higher performance
than SnSe for temperatures above 500 K. By extending the analysis to 900 K, we
obtained an ultrahigh value for the thermoelectric figure of merit (zT = 3.2)
at the optimal hole density of 4x10^19 cm^-3. Our work provides strong
motivation for continued experimental work focusing on improving the GeSe
doping efficiency in order to achieve this optimal hole density.

###Thermoelectric Properties of Mg doped Mercury Selenide HgSe|Y. Selmani,H. Labrim,A. Jabar,L. Bahmad###

Thermoelectric Properties of Mg doped Mercury Selenide HgSe. Using the density functional theory (DFT) in combination with Boltzmann
transport theory, the influence of Mg concentrations (x) doping on the
thermoelectric properties of Hg1-xMgxSe ternary alloys was systematically
investigated. The generalized gradient approximations of Perdew-Burke-Ernzerhof
(GGA-PBE) have been used to illustrate the exchange correlation potential.
Various thermoelectric transport parameters, such as the Seebeck coefficient
(S), the thermal conductivity over relaxation time, the electrical conductivity
over relaxation time, the power factor (PF) and the figure of merit (ZT) have
been deduced and discussed. The obtained results of thermoelectric properties
show that the studied materials can be useful for room temperature
thermoelectric devices. It is also found that Mg compositions can increase the
thermal efficiency of the HgSe alloy.

###Thermoelectric power factor of nanocomposite materials from two-dimensional quantum transport simulations|Samuel Foster,Mischa Thesberg,Neophytos Neophytou###

Thermoelectric power factor of nanocomposite materials from two-dimensional quantum transport simulations. Nanocomposites are promising candidates for the next generation of
thermoelectric materials since they exhibit extremely low thermal
conductivities as a result of phonon scattering on the boundaries of the
various material phases. The nanoinclusions, however, should not degrade the
thermoelectric power factor, and ideally should increase it, so that benefits
to the ZT figure of merit can be achieved. In this work we employ the
Non-Equilibrium Greens Function (NEGF) quantum transport method to calculate
the electronic and thermoelectric coefficients of materials embedded with
nanoinclusions. For computational effectiveness we consider two-dimensional
nanoribbon geometries, however, the method includes the details of geometry,
electron-phonon interactions, quantisation, tunneling, and the ballistic to
diffusive nature of transport, all combined in a unified approach. This makes
it a convenient and accurate way to understand electronic and thermoelectric
transport in nanomaterials, beyond semiclassical approximations, and beyond
approximations that deal with the complexities of the geometry. We show that
the presence of nanoinclusions within a matrix material offers opportunities
for only weak energy filtering, significantly lower in comparison to
superlattices, and thus only moderate power factor improvements. However, we
describe how such nanocomposites can be optimised to limit degradation in the
thermoelectric power factor and elaborate on the conditions that achieve the
aforementioned mild improvements. Importantly, we show that under certain
conditions, the power factor is independent of the density of nanoinclusions,
meaning that materials with large nanoinclusion densities which provide very
low thermal conductivities, can also retain large power factors and result in
large ZT figures of merit.

###Enhancing the thermoelectric performance of a HfS2 monolayer through valley engineering|H. Y. Lv,W. J. Lu,X. Luo,H. Y. Lu,X. B. Zhu,Y. P. Sun###

Enhancing the thermoelectric performance of a HfS2 monolayer through valley engineering. The electronic, phonon, and thermoelectric properties of a two-dimensional
HfS2 monolayer are investigated by using the first-principles calculations
combined with the Boltzmann transport theory. The band valleys of the HfS2
monolayer can be effectively tuned by the applied biaxial strain. The Seebeck
coefficient and therefore the peak value of the power factor (with the
relaxation time inserted) increase when the degeneracy of the band valleys is
increased by the strain. When no strain is applied, the HfS2 monolayer is an
excellent n-type thermoelectric material, while the thermoelectric performance
of the p-type doped one is poor. The applied tensile strain of 6% can increase
the room-temperature ZT value of the p-type doped system to 3.67, which is five
times larger than that of the unstrained one. The much more balanced ZT values
of the p- and n-type doping are favorable for fabrication of both p- and n-legs
of thermoelectric modules. Our results indicate that the thermoelectric
performance of the HfS2 monolayer can be greatly improved by the valley
engineering through the method of strain.

###Mg$_{3+δ}$Sb$_x$Bi$_{2-x}$ family: A promising substitute for the start-of-art n-type thermoelectric materials near room temperature|Rui Shu,Yecheng Zhou,Qi Wang,Zhijia Han,Yongbin Zhu,Yong Liu,Yuexing Chen,Meng Gu,Wei Xu,Yu Wang,Wenqing Zhang,Li Huang,Weishu Liu###

Mg$_{3+δ}$Sb$_x$Bi$_{2-x}$ family: A promising substitute for the start-of-art n-type thermoelectric materials near room temperature. Bi2Te3-xSex family has been the n-type start-of-the-art thermoelectric
materials near room temperatures (RT) for more than half-century, which
dominates the active cooling and novel waves harvesting application near RT.
However, the drawbacks of brittle nature and Te-containing restrict the further
applications exploring. Here, we show that a Mg3+{\delta}SbxBi2-x family
((ZT)avg =1.05) could be a promising substitute for the Bi2Te3-xSex family
((ZT)avg =0.9-1.0) in the temperature range of 50-250 {\deg}C based on the
comparable thermoelectric performance through a synergistic effect from the
tunable band gap using the alloy effect and the suppressible Mg-vacancy
formation using interstitial Mn dopant. The former is to shift the optimal
thermoelectric performance to near RT, and latter is helpful to partially
decouple the electrical transport and thermal transport in order to get an
optimal RT power factor. A positive temperature-dependence of band gap
suggested this family is also a superior medium-temperature thermoelectric
material for the significantly suppressed bipolar effect. Furthermore, a two
times higher mechanical toughness, compared with Bi2Te3-xSex family,
consolidates the promising substitute for the start-of-art n-type
thermoelectric materials near RT.

###Thermoelectricity of Tin Selenide Monolayers Across a Structural Phase Transition|John W. Villanova,Salvador Barraza-Lopez###

Thermoelectricity of Tin Selenide Monolayers Across a Structural Phase Transition. SnSe monolayers experience a temperature induced two-dimensional Pnm2$_1 \to$
P4/nmm structural transformation precipitated by the softening of vibrational
modes. The standard theoretical treatment of thermoelectricity---which relies
on a zero temperature phonon dispersion and on a zero temperature electronic
structure---is incapable of describing thermoelectric phenomena induced by
structural transformations. Relying on structural data obtained from {\em ab
initio} molecular dynamics calculations that is utilized in a non-standard way
to inform of electronic and vibrational transport coefficients, the present
work establishes a general route to understand thermoelectricity across phase
transitions. Similar to recent experimental observations pointing to an
overestimated thermoelectric figure of merit $ZT$ past the transition
temperature, our work indicates a smaller $ZT$ when compared to its value
predicted by the standard paradigm. Its decrease is related to the dramatic
changes in the electrical conductivity and lattice thermal conductivity as the
structural transformation ensues. Though exemplified on a SnSe monolayer, the
method does not have any built-in assumptions concerning dimensionality, and
thus applicable to arbitrary thermoelectric materials in one, two, and three
dimensions.

###Rhodium based half-Heusler alloys as possible optoelectronic and thermoelectric materials|Dhurba Raj Jaishi,Sujit Bati,Nileema Sharma,Bishnu Karki,Bishnu Prasad Belbase,Madhav Prasad Ghimire###

Rhodium based half-Heusler alloys as possible optoelectronic and thermoelectric materials. On the basis of density functional theory and semi-classical Boltzmann
theory, we have investigated the structural, elastic, electronic, optical and
thermoelectric properties of 18--valence electron count rhodium based
half-Heusler alloys focusing on RhTiP, RhTiAs, RhTiSb, and RhTiBi. The absence
of imaginary frequencies in the phonon dispersion curve for these system
verifies that they are structurally stable. RhTiP is ductile in nature, while
others are brittle. The alloys are found to be semiconducting with indirect
band gaps ranging from 0.94 to 1.01 eV. Our calculations suggest these
materials to have high absorption coefficient and optical conductivity in the
ultraviolet as well as visible region. While considering thermoelectricity, we
found that $p$--type doping is more favorable in improving the thermoelectric
properties. The calculated values of power factor with $p$-type doping are
comparable to some of the reported half-Heusler materials. The optimum figure
of merit \zt\ is $\sim1$ for RhTiBi suggesting it as a promising candidate for
thermoelectric applications while RhTiP, RhTiAs, and RhTiSb with optimum \zt \
values between 0.38 to 0.67 are possible candidates for use in thermoelectric
devices.

###Control of thermoelectric properties of phase-coherent molecular wires|Víctor M. García-Suarez,Colin J. Lambert,David Zs. Manrique,Thomas Wandlowski###

Control of thermoelectric properties of phase-coherent molecular wires. We demonstrate how redox control of intra-molecular quantum interference in
phase-coherent molecular wires can be used to enhance the thermopower (Seebeck
coefficient) S and thermoelectric figure of merit ZT of single molecules
attached to nanogap electrodes. Using first principles theory, we study the
thermoelectric properties of a family of nine molecules, which consist of
dithiol-terminated oligo(phenylene-ethynylenes) (OPEs) containing various
central units. Uniquely, one molecule of this family possesses a conjugated
acene-based central backbone attached via triple bonds to terminal sulfur atoms
bound to gold electrodes and incorporates a fully conjugated hydroquinone
central unit. We demonstrate that both S and the electronic contribution ZelT
to the figure of merit ZT can be dramatically enhanced by oxidizing the
hydroquinone to yield a second molecule, which possesses a cross-conjugated
anthraquinone central unit. This enhancement originates from the conversion of
the pi-conjugation in the former to cross-conjugation in the latter, which
promotes the appearance of a sharp anti-resonance at the Fermi energy.
Comparison with thermoelectric properties of the remaining seven conjugated
molecules demonstrates that such large values of S and ZelT are unprecedented.
We also evaluate the phonon contribution to the thermal conductance, which
allows us to compute the full figure of merit ZT = ZelT/(1 +
\k{appa}p/\k{appa}el), where \k{appa}p is the phonon contribution to the
thermal conductance and \k{appa}el is the electronic contribution. For
unstructured gold electrodes, \k{appa}p/\k{appa}el >> 1 and therefore
strategies to reduce \k{appa}p are needed to realise the highest possible
figure of merit.

###Enhanced Thermoelectric Properties of Dirac Semimetal Cd3As2|Tong Zhou,Cheng Zhang,Huisheng Zhang,Faxian Xiu,Zhongqin Yang###

Enhanced Thermoelectric Properties of Dirac Semimetal Cd3As2. We report an investigation of temperature- and doping-dependent
thermoelectric behaviors of topological semimetal Cd3As2. The electrical
conductivity, thermal conductivity, Seebeck coefficient, and figure of merit
(ZT) are calculated by using Boltzmann transport theory. The calculated
thermoelectric properties of the pristine Cd3As2 match well the experimental
results. The electron or hole doping, especially the latter, is found improving
much the thermoelectric behaviors of the material. The optimum merit ZT of
Cd3As2 with electron doping is found to be about 0.5 at T=700 K with n=1x1020
cm-3, much larger than the maximum experimental value obtained for the pristine
Cd3As2 (~0.15). For the p-type Cd3As2, the maximal value of the Seebeck
coefficient as a function of temperature increases apparently with the increase
of the hole doping concentration and its position shifts drastically towards
the lower temperature region compared to that of the n-type Cd3As2, leading to
the optimum merit ZT of about 0.5 obtained at low temperature of 500K (p=1x1020
cm-3) in the p-type Cd3As2.

###First-principles calculation of the thermoelectric figure of merit for [2,2]paracyclophane-based single-molecule junctions|Marius Bürkle,Thomas J. Hellmuth,Fabian Pauly,Yoshihiro Asai###

First-principles calculation of the thermoelectric figure of merit for [2,2]paracyclophane-based single-molecule junctions. Here we present a theoretical study of the thermoelectric transport through
{[}2,2{]}para\-cyclo\-phane-based single-molecule junctions. Combining
electronic and vibrational structures, obtained from density functional theory
(DFT), with nonequilibrium Green's function techniques, allows us to treat both
electronic and phononic transport properties at a first-principles level. For
the electronic part, we include an approximate self-energy correction, based on
the DFT+$\Sigma$ approach. This enables us to make a reliable prediction of all
linear response transport coefficients entering the thermoelectric figure of
merit $ZT$. Paracyclophane derivatives offer a great flexibility in tuning
their chemical properties by attaching different functional groups. We show
that, for the specific molecule, the functional groups mainly influence the
thermopower, allowing to tune its sign and absolute value. We predict that the
functionalization of the bare paracyclophane leads to a largely enhanced
electronic contribution $Z_{\mathrm{el}}T$ to the figure of merit.
Nevertheless, the high phononic contribution to the thermal conductance
strongly suppresses $ZT$. Our work demonstrates the importance to include the
phonon thermal conductance for any realistic estimate of the $ZT$ for
off-resonant molecular transport junctions. In addition, it shows the
possibility of a chemical tuning of the thermoelectric properties for a series
of available molecules, leading to equally performing hole- and
electron-conducting junctions based on the same molecular framework.

###First-principles prediction of extraordinary thermoelectric efficiency in superionic Li2SnX3(X=S,Se)|Enamul Haque,Claudio Cazorla,M. Anwar Hossain###

First-principles prediction of extraordinary thermoelectric efficiency in superionic Li2SnX3(X=S,Se). Thermoelectric materials create an electric potential when subject to a
temperature gradient and vice versa hence they can be used to harvest waste
heat into electricity and in thermal management applications. However, finding
highly efficient thermoelectrics with high figures of merit, zT$\geq$1, is very
challenging because the combination of high power factor and low thermal
conductivity is rare in materials. Here, we use first-principles methods to
analyze the thermoelectric properties of Li$_2$Sn$X_3$ ($X$=S,Se), a recently
synthesized class of lithium fast-ion conductors presenting high thermal
stability. In p-type Li$_2$Sn$X_3$, we estimate highly flat electronic valence
bands that render high Seebeck coefficients exceeding 400 ${\mu}$VK$^{-1}$ at
700K. In n-type Li$_2$Sn$X_3$, the electronic conduction bands are slightly
dispersive however the accompanying weak electron-acoustic phonon scattering
induces high electrical conductivity. The combination of high Seebeck
coefficient and electrical conductivity gives rise to high power factors,
reaching a maximum of 4 mWm$^{-1}$K$^{-2}$ in p-type Li$_2$SnS$_3$ and 8
mWm$^{-1}$K$^{-2}$ in n-type Li$_2$SnSe$_3$ at 300 K. Likewise, the thermal
conductivity in Li$_2$Sn$X_3$ is low as compared to conventional thermoelectric
materials, 2-5 Wm$^{-1}$K$^{-1}$ at room temperature. As a result, we estimate
a maximum zT = 1.05 in p-type Li$_2$SnS$_3$ at 700 K and an extraordinary 3.07
(1.5) in n-type Li$_2$SnSe$_3$ at the same temperature (300 K). Our findings of
huge zT in Li$_2$Sn$X_3$ suggest that lithium fast-ion conductors, typically
employed as electrolytes in solid-state batteries, hold exceptional promise as
thermoelectric materials.

###Thermoelectric properties of chalcopyrite type CuGaTe$_2$ and chalcostibite CuSbS$_2$|Vijay Kumar Gudelli,V. Kanchana,G. Vaitheeswaran,A. Svane,N. E. Christensen###

Thermoelectric properties of chalcopyrite type CuGaTe$_2$ and chalcostibite CuSbS$_2$. Electronic and transport properties of CuGaTe$_2$, a hole-doped ternary
copper based chalcopyrite type semiconductor, are studied using calculations
within the Density Functional Theory and solving the Boltzmann transport
equation within the constant relaxation time approximation. The electronic
bandstructures are calculated by means of the full-potential linear augmented
plane wave method, using the Tran-Blaha modified Becke-Johnson potential. The
calculated band gap of 1.23 eV is in agreement with the experimental value of
1.2 eV. The carrier concentration- and temperature dependent thermoelectric
properties of CuGaTe$_2$ are derived, and a figure of merit of $zT= 1.69$ is
obtained at 950 K for a hole concentration of $3.7\cdot10^{19}$ cm$^{-3}$, in
agreement with a recent experimental finding of $zT= 1.4$, confirming that
CuGaTe$_2$ is a promising material for high temperature thermoelectric
applications. The good thermoelectric performance of p-type CuGaTe$_2$ is
associated with anisotropic transport from a combination of heavy and light
bands. Also for CuSbS$_2$ (chalcostibite) a better performance is obtained for
p-type than for n-type doping. The variation of the thermopower as a function
of temperature and concentration suggests that CuSbS$_2$ will be a good
thermoelectric material at low temperatures, similarly to the isostructural
CuBiS$_2$ compound.

###Mitigating the Effect of Nanoscale Porosity on Thermoelectric Power Factor of Si|S. Aria Hosseini,Giuseppe Romano,P. Alex Greaney###

Mitigating the Effect of Nanoscale Porosity on Thermoelectric Power Factor of Si. The addition of porosity to thermoelectric materials can significantly
increase the figure of merit, ZT, by reducing the thermal conductivity.
Unfortunately, porosity is also detrimental to the thermoelectric power factor
in the numerator of the figure of merit ZT. In this manuscript we derive
strategies to recoup electrical performance in nanoporous Si by fine tuning the
carrier concentration and through judicious design of the pore size and shape
so as to provide energy selective electron filtering. In this study, we
considered phosphorus doped silicon containing discrete pores that are either
spheres, cylinders, cubes, or triangular prisms. The effects from these pores
are compared with those from extended pores with circular, square and
triangular cross sectional shape, and infinite length perpendicular to the
electrical current. A semiclassical Boltzmann transport equation is used to
model Si thermoelectric power factor. This model reveals three key results: The
largest enhancement in Seebeck coefficient occurs with cubic pores. The
fractional improvement is about 15% at low carrier concentration ($< 10^{20}\
\mathrm{1/cm^3}$) up to 60% at high carrier population with characteristic
length around $\sim 1\ \mathrm{nm}$. To obtain the best energy filtering effect
at room temperature, nanoporous Si needs to be doped to higher carrier
concentration than is optimal for bulk Si. Finally, in $n$-type Si
thermoelectrics the electron filtering effect that can be generated with
nanoscale porosity is significantly lower than the ideal filtering effect;
nevertheless, the enhancement in the Seebeck coefficient that can be obtained
is large enough to offset the reduction in electrical conductivity caused by
porosity.

###Extraordinary thermoelectric performance of ABaX compared to Bi$_2$Te$_3$|Enamul Haque###

Extraordinary thermoelectric performance of ABaX compared to Bi$_2$Te$_3$. Thermoelectric materials can generate electricity directly utilizing heat and
thus, they are considered to be eco-friendly energy resources. The
thermoelectric efficiency at low temperatures is impractically small, except
only a few bulk materials (Bi$_2$Te$_3$ and its alloys). Here, I predict two
new thermoelectric materials, LiBaSb and NaBaBi, with excellent transport
properties at low-medium temperature by using the first-principles method. The
relatively low density of states near Fermi level, highly non-parabolic bands,
and almost two times wider bandgap of NaBaBi lead to almost two times higher
anisotropic power factor at 300K than that of Bi2Te3. On the other side, almost
similar phonon density of states and anharmonicity of NaBaBi cause almost
identical lattice thermal conductivity (but it is much higher in LiBaSb). These
effects make it a superior thermoelectric material, with a predicted
cross-plane (in-plane) ZT ~2 (~1) at 300 K for both n- and p-type carriers,
even higher (~2.5 for p-type) at 350K. On the other hand, the isotropic maximum
ZT of NaBaBi is ~1.2 and 1.6 at 350K for n and p-type carriers, respectively.
However, LiBaSb is less suitable for low-temperature TE applications, because
of its relatively wider bandgap and high lattice thermal conductivity.

###Non-Wiedemann-Franz Behavior of the Thermal Conductivity of Organic Semiconductors|Dorothea Scheunemann,Martijn Kemerink###

Non-Wiedemann-Franz Behavior of the Thermal Conductivity of Organic Semiconductors. Organic semiconductors have attracted increasing interest as thermoelectric
converters in recent years due to their intrinsically low thermal conductivity
compared to inorganic materials. This boom has led to encouraging practical
results, in which the thermal conductivity has predominantly been treated as an
empirical number. However, in an optimized thermoelectric material, the
electronic component can dominate the thermal conductivity in which case the
figure of merit $ZT$ becomes a function of thermopower and Lorentz factor only.
Hence design of effective organic thermoelectric materials requires
understanding the Lorenz number. Here, analytical modeling and kinetic Monte
Carlo simulations are combined to study the effect of energetic disorder and
length scales on the correlation of electrical and thermal conductivity in
organic semiconductor thermoelectrics. We show that a Lorenz factor up to a
factor $\sim 5$ below the Sommerfeld value can be obtained for weakly
disordered systems, in contrast with what has been observed for materials with
band transport. Although the electronic contribution dominates the thermal
conductivity within the application-relevant parameter space, reaching ZT>1
would require to minimize both the energetic disorder but also the lattice
thermal conductivity to values below $\kappa_\text{lat}<0.2$W/mK.

###Spin Fluctuations Yield zT Enhancement in Ferromagnets|Md Mobarak Hossain Polash,Daryoosh Vashaee###

Spin Fluctuations Yield zT Enhancement in Ferromagnets. Thermal fluctuation of local magnetization in magnetic metals intercoupled
with charge carriers and phonons offers a path to enhance thermoelectric
performance. The thermopower enhancement by spin fluctuations (SF) has been
observed before. However, the crucial evidence for enhancing
thermoelectric-figure-of-merit (zT) by SF has not been reported until now. Here
we report evidence for such enhancement in the ferromagnetic CrTe. The SF leads
to nearly 80% zT enhancement in ferromagnetic CrTe near and below TC~335 K. The
ferromagnetism in CrTe is originated from the collective electronic and
localized magnetic moments. The field-dependent transport properties
demonstrate the profound impact of SF on the electrons and phonons. The SF
simultaneously enhances the thermopower and reduces the thermal conductivity.
Under an external magnetic field, the enhancement in thermopower is suppressed,
and the thermal conductivity is enhanced, evidencing the existence of a strong
SF near and below TC. The anomalous thermoelectric transport properties are
analyzed based on theoretical models, and a good agreement with experimental
data is found. Furthermore, the detailed analysis proves an insignificant
impact from spin-wave contribution to the transport properties. This study
contributes to the fundamental understanding of spin fluctuation for designing
high-performance spin-driven thermoelectric materials.

###Doping as a tuning mechanism for magneto-thermoelectric effects to improve zT in polycrystalline NbP|Eleanor F. Scott,Katherine A. Schlaak,Poulomi Chakraborty,Chenguang Fu,Satya N. Guin,Safa Khodabakhsh,Ashley E. Paz y Puente,Claudia Felser,Brian Skinner,Sarah J. Watzman###

Doping as a tuning mechanism for magneto-thermoelectric effects to improve zT in polycrystalline NbP. Weyl semimetals combine topological and semimetallic effects, making them
candidates for interesting and effective thermoelectric transport properties.
Here, we present experimental results on polycrystalline NbP, demonstrating the
simultaneous existence of a large Nernst effect and a large magneto-Seebeck
effect, which is typically not observed in a single material at the same
temperature. We compare transport results from two polycrystalline samples of
NbP with previously published work, observing a shift in the temperature at
which the maximum Nernst and magneto-Seebeck thermopowers occur, while still
maintaining thermopowers of similar magnitude. Theoretical modeling shows how
doping strongly alters both the Seebeck and Nernst magneto-thermopowers by
shifting the temperature-dependent chemical potential, and the corresponding
calculations provide a consistent interpretation of our results. Thus, we offer
doping as a tuning mechanism for shifting magneto-thermoelectric effects to
temperatures appropriate for device applications, improving zT at desirable
operating temperature. Furthermore, the simultaneous presence of both a large
Nernst and magneto-Seebeck thermopower is uncommon and offers unique device
advantages if the thermopowers are used additively. Here, we also propose a
unique thermoelectric device which would collectively harness the large Nernst
and magneto-Seebeck thermopowers to greatly enhance the output and zT of
conventional thermoelectric devices.

###Thermoelectric transport through a quantum nanoelectromechanical system and its backaction|Hangbo Zhou,Juzar Thingna,Jian-Sheng Wang,Baowen Li###

Thermoelectric transport through a quantum nanoelectromechanical system and its backaction. We present a comprehensive study of thermoelectric transport properties of a
quantum nanoelectromechanical system (NEMS) described by a
single-electron-transistor (SET) coupled to a quantum nanomechanical resonator
(NR). The effects of a quantum NR on the electronic current are investigated
with special emphasis on how the SET-NR coupling strength plays a role in such
a NEMS. We find that the SET-NR coupling is not only able to suppress or
enhance the thermoelectric current but can also switch its direction. The
effect of the NR on the thermoelectric coefficients of the SET are studied and
we find that even a small SET-NR coupling could dramatically suppress the
figure of merits ZT . Lastly, we investigate the backaction of electronic
current on the NR and possible routes of heating or cooling the NR are
discussed. We find that by appropriately tuning the gate voltage the backaction
can be eliminated, which could find possible applications to enhance the
sensitivity of detection devices.

###Enhancing thermoelectric figure-of-merit by low-dimensional electrical transport in phonon-glass crystals|Xue-Ya Mi,Xiaoxiang Yu,Kai-Lun Yao,Xiaoming Huang,Nuo Yang,Jing-Tao Lü###

Enhancing thermoelectric figure-of-merit by low-dimensional electrical transport in phonon-glass crystals. Low-dimensional electronic and glassy phononic transport are two important
ingredients of highly-efficient thermoelectric material, from which two
branches of the thermoelectric research emerge. One focuses on controlling
electronic transport in the low dimension, while the other on multiscale phonon
engineering in the bulk. Recent work has benefited much from combining these
two approaches, e.g., phonon engineering in low-dimensional materials. Here, we
propose to employ the low-dimensional electronic structure in bulk phonon-glass
crystal as an alternative way to increase the thermoelectric efficiency.
Through first-principles electronic structure calculation and classical
molecular dynamics simulation, we show that the $\pi$-$\pi$ stacking
Bis-Dithienothiophene molecular crystal is a natural candidate for such an
approach. This is determined by the nature of its chemical bonding. Without any
optimization of the material parameter, we obtain a maximum room-temperature
figure of merit, $ZT$, of $1.48$ at optimal doping, thus validating our idea.

###Thermoelectric properties of Sr0.61Ba0.39Nb2O6-δ ceramics annealed in different oxygen-reduction conditions|Y. Li,J. Liu,C. L. Wang,W. B. Su,Y. H. Zhu,J. C. Li,L. M. Mei###

Thermoelectric properties of Sr0.61Ba0.39Nb2O6-δ ceramics annealed in different oxygen-reduction conditions. The thermoelectric properties of Sr0.61Ba0.39Nb2O6 ceramics, reduced in
different conditions, were investigated in the temperature region from 323 K to
1073 K. The electrical transport behaviors of the samples are dominated by the
thermal-activated polaron hopping, the Fermi glass behavior, and the Anderson
localized behavior from low temperatures to high temperatures, respectively.
The lattice thermal conductivity presents a plateau at high temperatures,
indicating a glass-like thermal conduction behavior. Both the thermoelectric
power factor and the thermal conductivity increase with the increasing degree
of oxygen-reduction. Taking these two factors into account, the
oxygen-reduction can still contribute to promoting the thermoelectric figure of
merit. The highest ZT value (~0.19 at 1073 K) is obtained in the heaviest
oxygen reduced sample.

###Theoretical study of the thermoelectric properties of SiGe nanotubes|J. Wei,H. J. Liu,X. J. Tan,L. Cheng,J. Zhang,D. D. Fan,J. Shi,X. F. Tang###

Theoretical study of the thermoelectric properties of SiGe nanotubes. The thermoelectric properties of two typical SiGe nanotubes are investigated
using a combination of density functional theory, Boltzmann transport theory,
and molecular dynamics simulations. Unlike carbon nanotubes, these SiGe
nanotubes tend to have gear-like geometry, and both the (6, 6) and (10, 0)
tubes are semiconducting with direct band gaps. The calculated Seebeck
coefficients as well as the relaxation time of these SiGe nanotubes are
significantly larger than those of bulk thermoelectric materials. Together with
smaller lattice thermal conductivity caused by phonon boundary and alloy
scattering, these SiGe nanotubes can exhibit very good thermoelectric
performance. Moreover, there are strong chirality and temperature dependence of
the ZT values, which can be optimized to 4.9 at room temperature and further
enhanced to 5.4 at 400 K for the armchair (6, 6) tube.

###Thermoelectric properties of two-dimensional slabs of Ba8Ga16Ge30 from first principles|Deepa Kasinathan,Vicente Pacheco-Espejel,Helge Rosner###

Thermoelectric properties of two-dimensional slabs of Ba8Ga16Ge30 from first principles. Thermoelectric effects enable the direct conversion between thermal and
electrical energy and provide an alternative route for power generation and
refrigeration. The clathrate Ba8Ga16Ge30 has the highest figure of merit (ZT ~
1) among other members in the family of type-I inorganic clathrates.
Enhancement of the thermoelectric properties have been observed in multilayered
superlattices, quantum wires and in nanostructured materials, either due to the
increase in power-factor (S^{2}\sigma) or due to the reduction of lattice
thermal conductivity (\kappa). Here, we investigate the thermoelectric
properties of two-dimensional slabs with varying thickness of Ba8Ga16Ge30 using
semi-classical Boltzmann transport theory with constant scattering
approximation. We observe that, there exists a delicate balance between the
electrical conductivity and the electronic part of the thermal conductivity in
reduced dimensions and the insights from these results can directly be used to
control particle size in nanostructuring experiments. The calculated properties
are consistent with the recent, first measurements on bulk nanostructured
samples.

###Non-linear impedance spectroscopy applied to thermoelectric measurements: beyond the ZT estimation|Etienne Thiébaut,François Pesty,Christophe Goupil,Guillaume Guegan,Philippe Lecoeur###

Non-linear impedance spectroscopy applied to thermoelectric measurements: beyond the ZT estimation. Thermoelectric measurement of the dimensionless zT parameter requires
multiple physical quantities to be measured, therefore there is great interest
to find an experimental setup capable of measuring all these properties at
once. Previous works on impedance spectroscopy have shown promising results in
this direction, however, this technique does not lead to a complete
characterization of the thermoelectric system without additional measurement.
In order to extend impedance spectroscopy, we have investigated the measurement
of the non-linear harmonic response of a Peltier device. The experiments are
analyzed using an analytic model obtained by solving the heat equation in the
frequency regime. Our work shows that fitting the experimental response of the
system in the harmonic regime can lead to a complete characterization of the
thermoelectric properties without the need of additional measurement.

###High figure-of-merit in the heavy-fermion UN2 system for radioisotope thermoelectric applications|Z. Z. Zhou,D. D. Fan,H. J. Liu,J. Liu###

High figure-of-merit in the heavy-fermion UN2 system for radioisotope thermoelectric applications. The design of uranium-based thermoelectric materials presents a novel and
intriguing strategy for directly converting nuclear heat into electrical power.
Using high-level first-principles approach combined with accurate solution of
Boltzmann transport equation, we demonstrate that a giant n-type power factor
of 13.8 mW/mK^2 and a peak ZT value of 2.2 can be realized in the heavy-fermion
UN2 compound at 700 K. Such promising thermoelectric performance arises from
the large degeneracy (Nv=14) of heavy conduction band coupled with weak
electron-phonon interactions, which is in principle governed by the strong
Coulomb correlation among the partially filled U-5f electrons in the
face-centered cubic structure. Collectively, our theoretical work suggests that
the energetic UN2 is an excellent alternative to efficient radioisotope power
conversion, which also uncovers an underexplored area for thermoelectric
research.

###Performance analysis of an interacting quantum dot thermoelectric system|Bhaskaran Muralidharan,Milena Grifoni###

Performance analysis of an interacting quantum dot thermoelectric system. We analyze the nanocaloritronic performance of an interacting quantum dot
that is subject to an applied bias and an applied temperature gradient. It is
now well known that, in the absence of phonon contribution, a weakly coupled
non-interacting quantum dot can operate at thermoelectric efficiencies
approaching the Carnot limit. However, it has also been recently pointed out
that such peak efficiencies can only be achieved when operated in the
reversible limit, with a vanishing current and hence a vanishing power output.
In this paper, we point out three fundamental results affecting the
thermoelectric performance due to the inclusion of Coulomb interactions: a) The
reversible operating point carries zero efficiency, b) operation at finite
power output is possible even at peak efficiencies approaching the Carnot
value, and c) the evaluated trends of the the maximum efficiency deviate
considerably from the conventional {\it{figure of merit}} $zT$ based result.
Finally, we also analyze our system for thermoelectric operation at maximum
power output.

###Suppression of vacancies boosts thermoelectric performance in type-I clathrates|Xinlin Yan,Matthias Ikeda,Long Zhang,Ernst Bauer,Peter Rogl,Gerald Giester,Andrey Prokofiev,Silke Paschen###

Suppression of vacancies boosts thermoelectric performance in type-I clathrates. Intermetallic type-I clathrates continue to attract attention as promising
thermoelectric materials. Here we present structural and thermoelectric
properties of single crystalline Ba8(Cu,Ga,Ge,v)46, where v denotes a vacancy.
By single crystal X-ray diffraction on crystals without Ga we find clear
evidence for the presence of vacancies at the 6c site in the structure. With
increasing Ga content, vacancies are successively filled. This increases the
charge carrier mobility strongly, even within a small range of Ga substitution,
leading to reduced electrical resistivity and enhanced thermoelectric
performance. The largest figure of merit ZT =0.9 at 900 K is found for a single
crystal of approximate composition Ba8Cu4.6Ga1.0Ge40.4. This value, that may
further increase at higher temperatures, is one of the largest to date found in
transition metal element-based clathrates.

###Quantum thermoelectrics based on 2-D Semi-Dirac materials|Alestin Mawrie,Bhaskaran Muralidharan###

Quantum thermoelectrics based on 2-D Semi-Dirac materials. We show that a gap parameter can fully describe the merging of Dirac cones in
semi-Dirac materials from $K$- and $K^\prime$-points into the common $M$-point
in the Brillouin zone. We predict that the gap parameter manifests itself by
enhancing the thermoelectric figure of merit $zT$ as the chemical potential
crosses the gap followed by a sign change in the Seebeck coefficient around the
same point. Subsequently, we show that there is also a trade-off feature
between the maximum power delivered and the efficiency when the chemical
potential crosses the gap parameter. An optimal operating point that minimizes
the power-efficiency trade-off is consequently singled out for the best
thermoelectric performance. Our work paves the way for the use of 2D semi-Dirac
materials for thermoelectric applications.

###High Thermoelectric Performance in Two-Dimensional Tellurium: An Ab Initio Study|Zhibin Gao,Gang Liu,Jie Ren###

High Thermoelectric Performance in Two-Dimensional Tellurium: An Ab Initio Study. In 2016, bulk tellurium was experimentally observed as a remarkable
thermoelectric material. Recently, two-dimensional (2D) tellurium, called
tellurene, has been synthesized and has exhibited unexpected electronic
properties compared with the 2D MoS$_2$. They have also been fabricated into
air-stable and high efficient field-effect transistors. There are two stable 2D
tellurene phases. One ($\beta$-Te) has been confirmed with an ultralow lattice
thermal conductivity ($\kappa_L$). However, the study of the transport
properties of the other more stable phase, $\alpha$-Te, is still lacking. Here,
we report the thermoelectric performance and phonon properties of $\alpha$-Te
using Boltzmann transport theory and first principle calculations. A maximum ZT
value of 0.83 is achieved under reasonable hole concentration, suggesting that
the monolayer $\alpha$-Te is a potential competitor in the thermoelectric
field.

###Thermoelectricity modeling with cold dipole atoms in Aubry phase of optical lattice|Oleg V. Zhirov,José Lages,Dima L. Shepelyansky###

Thermoelectricity modeling with cold dipole atoms in Aubry phase of optical lattice. We study analytically and numerically the thermoelectric properties of a
chain of cold atoms with dipole-dipole interactions placed in an optical
periodic potential. At small potential amplitudes the chain slides freely that
corresponds to the Kolmogorov-Arnold-Moser phase of integrable curves of a
symplectic map. Above a certain critical amplitude the chain is pinned by the
lattice being in the cantori Aubry phase. We show that the Aubry phase is
characterized by exceptional thermoelectric properties with the figure of merit
ZT = 25 being ten times larger than the maximal value reached in material
science experiments. We show that this system is well accessible for
magneto-dipole cold atom experiments that opens new prospects for
investigations of thermoelectricity.

###First-Principles Study of Strain Effect on Thermoelectric Properties of LaP and LaAs|Chia-Min Lin,Wei-Chih Chen,Cheng-Chien Chen###

First-Principles Study of Strain Effect on Thermoelectric Properties of LaP and LaAs. Rare-earth monopnictides have attracted much attention due to their unusual
electronic and topological properties for potential device applications. Here,
we study rock-salt structured lanthanum monopnictides LaX (X = P, As) by
density functional theory (DFT) simulations. We show systematically that a
meta-GGA functional combined with scissor correction can efficiently and
accurately compute electronic structures on a fine DFT $k$-grid, which is
necessary for converging thermoelectric calculations. We also show that strain
engineering can effectively improve thermoelectric performance. Under the
optimal condition of 2% tensile strain and carrier concentration
$n=3\times10^{20}~\textrm{cm}^{-3}$, LaP at temperature 1200 K can achieve a
figure of merit $ZT$ value $>2$, which is enhanced by 90% compared to the
unstrained value. With carrier doping and strain engineering, lanthanum
monopnictides thereby could be promising high-temperature thermoelectric
materials.

###Electronic Structure and Thermoelectric Properties of Half-Heusler Alloys NiTZ|Dhurba R. Jaishi,Nileema Sharma,Bishnu Karki,Bishnu P. Belbase,Rajendra P. Adhikari,Madhav Prasad Ghimire###

Electronic Structure and Thermoelectric Properties of Half-Heusler Alloys NiTZ. We have investigated the electronic and thermoelectric properties of
half-Heusler alloys NiTZ (T = Sc, and Ti; Z = P, As, Sn, and Sb) having 18
valence electron. Calculations are performed by means of density functional
theory and Boltzmann transport equation with constant relaxation time
approximation, validated by NiTiSn. The chosen half-Heuslers are found to be an
indirect band gap semiconductor, and the lattice thermal conductivity is
comparable with the state-of-the-art thermoelectric materials. The estimated
power factor for NiScP, NiScAs, and NiScSb reveals that their thermoelectric
performance can be enhanced by appropriate doping rate. The value of ZT found
for NiScP, NiScAs, and NiScSb are 0.46, 0.35, and 0.29, respectively at 1200 K.

###First-principles investigations of the electronic, magnetic and thermoelectric properties of VTiRhZ (Z= Al, Ga, In) Quaternary Heusler Alloys|Hind Alqurashi,Raad Haleoot,Bothina Hamad###

First-principles investigations of the electronic, magnetic and thermoelectric properties of VTiRhZ (Z= Al, Ga, In) Quaternary Heusler Alloys. Calculations using density functional theory (DFT) were performed to
investigate the structural, dynamical, mechanical, electronic, magnetic, and
thermoelectric properties of VTiRhZ (Z = Al, Ga, In) alloys. The most stable
structure of these alloys was found to be the type-I configuration. Using
GGA-PBE functional, VTiRhGa, and VTiRhIn alloys are predicted as half-metallic
ferromagnets with a 100% spin-polarization and a total magnetic moment of
3{\mu}B, which is promising for spintronic applications. The thermoelectric
properties and lattice thermal conductivity of VTiRhZ alloys were obtained
using the Boltzmann transport theory within the constant relaxation time and
Slack equation, respectively. The figure-of-merit (ZT) values of VTiRhAl,
VTiRhGa, and VTiRhIn alloys were found to be 0.96, 0.88 and 0.64, respectively,
which are promising for future thermoelectric applications.

###Bismuth based Half Heusler Alloys with giant thermoelectric figure of merit|Vikram,Jiban Kangsabanik,Enamullah,Aftab Alam###

Bismuth based Half Heusler Alloys with giant thermoelectric figure of merit. Half Heusler (HH) thermoelectric alloys provide a wide platform to choose
materials with non-toxic and earth abundant elements. This article presents an
ab-initio theoretical evaluation of electrical and thermal transport properties
of three Bismuth-based most promising thermoelectric alloys, selected out of 54
stable HH compounds. These are brand new compounds which are recently proposed
to be stable (Nature Chem. 7, 308 (2015)) and may have interesting properties.
The calculated band structure of the three compounds, namely HfRhBi, ZrIrBi and
ZrRhBi, served as a hint for their promising thermoelectric properties. To gain
confidence on the theoretical predictions of these unreported systems, we first
checked our calculated results for a well studied similar compound, ZrNiSn, and
showed reasonable agreement with the measured ones. HfRhBi and ZrIrBi turn out
to be narrow band gap while ZrRhBi is a moderate band gap semiconductor. A
detailed study of the carrier concentration and temperature dependance of the
Seebeck coefficient (S), Power factor (S$^2 \sigma$), lattice ($\kappa_L$) and
electronic ($\kappa_e$) thermal conductivity and hence the figure of merit (ZT)
is carried out. In contrast to most promising known thermoelectric materials,
we found high power factor for these materials (highest S$^2 \sigma\sim$17.36
mWm$^{-1}$K$^{-2}$ for p-type ZrIrBi). All the three systems (specially p-type)
show high figure of merit, with ZT value as high as 0.45 for ideal crystal.
Maximum ZT and the corresponding optimal n- and p-type doping concentrations
($n_c$) are calculated for all the three compounds, which shall certainly pave
guidance to future experimental work.

###Thermoelectric Properties of Scaled Silicon Nanowires Using the sp3d5s*-SO Atomistic Tight-Binding Model and Boltzmann Transport|Neophytos Neophytou,Hans Kosina###

Thermoelectric Properties of Scaled Silicon Nanowires Using the sp3d5s*-SO Atomistic Tight-Binding Model and Boltzmann Transport. As a result of suppressed phonon conduction, large improvements of the
thermoelectric figure of merit, ZT, have been recently reported for
nanostructures compared to the raw materials' ZT values. It has also been
suggested that low dimensionality can improve a device's power factor as well,
offering a further enhancement. In this work the atomistic
sp3d5s*-spin-orbit-coupled tight-binding model is used to calculate the
electronic structure of silicon nanowires (NWs). The linearized Boltzmann
transport theory is applied, including all relevant scattering mechanisms, to
calculate the electrical conductivity, the Seebeck coefficient, and the
thermoelectric power factor. We examine n-type nanowires of diameters of 3nm
and 12nm, in [100], [110], and [111] transport orientations at different
carrier concentrations. Using experimental values for the lattice thermal
conductivity in nanowires, the expected ZT value is computed. We find that at
room temperature, although scaling the diameter below 7nm can be beneficial to
the power factor due to banstructure changes alone, at those dimensions
enhanced phonon and surface roughness scattering degrades the conductivity and
reduces the power factor.

###Thermoelectric Properties of Nanocomposite Heavy Fermion CeCu6|Mani Pokharel,Tulashi Dahal,Zhifeng Ren,Cyril Opeil###

Thermoelectric Properties of Nanocomposite Heavy Fermion CeCu6. Samples of heavy fermion compound CeCu6 were prepared by hot-press technique.
Temperature-dependent (5-300 K) thermoelectric transport properties of the
samples were measured. The dimensionless figure-of-merit (ZT) was optimized by
varying the hot-pressing temperature. Our measurements of thermal conductivity
show that the lowest hot pressing temperature (450 C) produces the lowest
thermal conductivity. Electrical resistivity increases significantly while the
Seebeck coefficient decreases with decrease in the hot pressing temperature. As
the hot-pressing temperature decreases, electronic contribution to the total
thermal conductivity decreased more rapidly than the lattice contribution did.
As a result, for lower hot-pressing temperature the gain in thermal
conductivity reduction was offset by the loss in power factor. Our ZT
calculations show a broad peak with a maximum value of 0.024 at 60 K for the
sample hot pressed at 800 C. The pronounced low-temperature ZT peak emphasizes
the importance of this heavy fermion system as a potential p-type
thermoelectric for solid state cooling applications.

###Thermoelectric Effects and Topological Insulators|Yong Xu###

Thermoelectric Effects and Topological Insulators. The recent discovery of topological insulator (TI) offers new opportunities
for the development of thermoelectrics, because many TIs (like Bi$_2$Te$_3$)
are excellent thermoelectric (TE) materials. In this review, we will first
describe the general TE properties of TIs and show that the coexistence of the
bulk and boundary states in TIs introduces unusual TE properties, including
strong size effects and anomalous Seebeck effect. Importantly, the TE figure of
merit $zT$ of TIs is no longer an intrinsic property, but depends strongly on
the geometric size. The geometric parameters of two-dimensional TIs can be
tuned to enhance $zT$ to be significantly greater than 1. Then a few
proof-of-principle experiments on three-dimensional TIs will be discussed,
which observed unconventional TE phenomena that are closely related to the
topological nature of the materials. However, current experiments indicate that
the metallic surface states, if their advantage of high mobility is not fully
utilized, would be detrimental to TE performance. Finally we provide an outlook
for future work on topological materials, which offers great possibilities to
discover exotic TE effects and may lead to significant breakthroughs in
improving $zT$.

###Giant thermoelectric effect in graphene-based topological insulators with nanopores|Po-Hao Chang,Mohammad Saeed Bahramy,Naoto Nagaosa,Branislav K. Nikolic###

Giant thermoelectric effect in graphene-based topological insulators with nanopores. Designing thermoelectric materials with high figure of merit $ZT=S^2 G
T/\kappa$ requires fulfilling three often irreconcilable conditions, i.e., the
high electrical conductance $G$, small thermal conductance $\kappa$ and high
Seebeck coefficient $S$. Nanostructuring is one of the promising ways to
achieve this goal as it can substantially suppress lattice contribution to
$\kappa$. However, it may also unfavorably influence the electronic transport
in an uncontrollable way. Here we theoretically demonstrate that this issue can
be ideally solved by fabricating graphene nanoribbons with heavy adatoms and
nanopores. These systems, acting as a two-dimensional topological insulator
with robust helical edge states carrying electrical current, yield a highly
optimized power factor $S^2G$ per helical conducting channel. Concurrently,
their array of nanopores impedes the lattice thermal conduction through the
bulk. Using quantum transport simulations coupled with first-principles
electronic and phononic band structure calculations, the thermoelectric figure
of merit is found to reach its maximum $ZT \simeq 3$ at $T \simeq 40$ K. This
paves a way to design high-$ZT$ materials by exploiting the nontrivial topology
of electronic states through nanostructuring.

###Enhancement of the electronic thermoelectric properties of bulk strained silicon-germanium alloys using the scattering relaxation times from first principles|F. Murphy-Armando###

Enhancement of the electronic thermoelectric properties of bulk strained silicon-germanium alloys using the scattering relaxation times from first principles. We use first-principles electronic structure methods to calculate the
electronic thermoelectric properties (i.e. due to electronic transport only) of
single-crystalline bulk $n$-type silicon-germanium alloys vs Ge composition,
temperature, doping concentration and strain. We find excellent agreement to
available experiments for the resistivity, mobility and Seebeck coefficient.
These results are combined with the experimental lattice thermal conductivity
to calculate the thermoelectric figure of merit $ZT$, finding very good
agreement with experiment. We predict that 3% tensile hydrostatic strain
enhances the $n$-type $ZT$ by 50% at carrier concentrations of $n=10^{20}$
cm$^{-3}$ and temperature of $T=1200K$. These enhancements occur at different
alloy compositions due to different effects: at 50% Ge composition the
enhancements are achieved by a strain induced decrease in the Lorenz number,
while the power factor remains unchanged. These characteristics are important
for highly doped and high temperature materials, in which up to 50% of the heat
is carried by electrons. At 70% Ge the increase in $ZT$ is due to a large
increase in electrical conductivity produced by populating the high mobility
$\Gamma$ conduction band valley, lowered in energy by strain.

###Effect of electron-phonon scattering, pressure and alloying on the thermoelectric performance of TmCu$_3$Ch$_4$ (Tm=V, Nb, Ta; Ch=S, Se, Te)|Enamul Haque###

Effect of electron-phonon scattering, pressure and alloying on the thermoelectric performance of TmCu$_3$Ch$_4$ (Tm=V, Nb, Ta; Ch=S, Se, Te). The demand for green energy increases day by day due to environmental concern
and thermoelectric (TE) materials are one of the eco-friendly energy resources.
Few authors reported high TE performance in TmCu$_3$Ch$_4$, reaching the figure
of merit (ZT) above 2 at 1000K, from first-principles calculations neglecting
electron-phonon scattering, spin-orbit coupling effect (SOC), and
energy-dependent carrier lifetime. Here, thermoelectric transport properties of
TmCu$_3$Ch$_4$ are reinvestigated through considering these parameters, and
significant discrepancies are found. The ZT of p-type TaCu$_3$Te$_4$ can reach
~3 at 1000K among these compounds due to its low lattice thermal conductivity
($\kappa_l$) (0.38 W m-1 K-1). Interestingly, the value of $\kappa_l$ is
reduced to 0.17 W m-1 K-1 through 1 GPa pressure while the power factor is
slightly improved due to bandgap reduction, leading to an extraordinary ZT~5.5
at 1000K. Although the substitution of Se causes a slight reduction of
$\kappa_l$ to ~0.3 W m-1 K-1, the power factor is reduced significantly due to
the dramatic reduction of DOS near Fermi level, which leads to lower the
Seebeck coefficient largely and increase electrical conductivity slightly.

###High thermoelectric performance in metastable phase of silicon: a first-principles study|Yongchao Rao,C. Y. Zhao,Shenghong Ju###

High thermoelectric performance in metastable phase of silicon: a first-principles study. In this work, both thermal and electrical transport properties of
diamond$-$cubic Si (Si$-$I) and metastable R8 phase of Si (Si$-$XII) are
comparatively studied by using first$-$principles calculations combined with
Boltzmann transport theory. The metastable Si$-$XII shows one magnitude lower
lattice thermal conductivity than stable Si$-$I from 300 to 500~K, attributed
from the stronger phonon scattering in three$-$phonon scattering processes of
Si$-$XII. For the electronic transport properties, although Si$-$XII with
smaller band gap (0.22 eV) shows lower Seebeck coefficient, the electrical
conductivities of anisotropic $n$$-$type Si$-$XII show considerable values
along $x$ axis due to the small effective masses of electron along this
direction. The peaks of thermoelectric figure of merit ($ZT$) in $n$$-$type
Si$-$XII are higher than that of $p$$-$type ones along the same direction.
Owing to the lower lattice thermal conductivity and optimistic electrical
conductivity, Si$-$XII exhibits larger optimal $ZT$ compared with Si$-$I in
both $p$$-$ and $n$$-$type doping. For $n$$-$type Si$-$XII, the optimal $ZT$
values at 300, 400, and 500 K can reach 0.24, 0.43, and 0.63 along $x$ axis at
carrier concentration of $2.6\times10^{19}$, $4.1\times10^{19}$, and
$4.8\times10^{19}$~cm$^{-3}$, respectively. The reported results elucidate that
the metastable Si could be integrated to the thermoelectric power generator.

###Thermoelectric Properties of a Semiconductor Quantum Dot Chain Connected to Metallic Electrodes|David M. -T. Kuo,Yia-Chung Chang###

Thermoelectric Properties of a Semiconductor Quantum Dot Chain Connected to Metallic Electrodes. The thermoelectric properties of a semiconduct quantum dot chain (SQDC)
connected to metallic electrodes are theoretically investigated in the Coulomb
blockade regime. An extended Hubbard model is employed to simulate the SQDC
system consisted of {\color{blue}N=2,3,4, and 5} quantum dots (QDs). The charge
and heat currents are calculated in the framework of Keldysh Green's function
technique. We obtained a closed-form Landauer expression for the transmission
coefficient of the SQDC system with arbitrary number of QDs by using the method
beyond mean-field theory. The electrical conductance ($G_e$), Seebeck
coefficient (S), thermal conductance, and figure of merit (ZT) are numerically
calculated and analyzed in the linear response regime. When thermal conductance
is dominated by phonon carriers, the optimization of ZT is determined by the
power factor ($pF=S^2G_e$). We find that the optimization of ZT value favors
the following conditions:(1) QDs with low energy level fluctuations, (2) QD
energy levels lie above the Fermi level of electrodes, (3) $\Gamma < t_c \ll
U_0$, where $t_c$, $U_0$, and $\Gamma$ are electron interdot hopping strength,
on-site electron Coulomb interaction, and tunneling rate, respectively, and (4)
$\Gamma_L=\Gamma_R$ with $\Gamma_L+\Gamma_R$ kept constant, where $\Gamma_L
(\Gamma_R)$ is the left (right) tunneling rate. It is predicted that high ZT
values can be achieved by tailoring above conditions.

###Half Heusler Alloys for Efficient Thermoelectric Power Conversion|L. Chen,X. Zeng,T. M. Tritt,S. J. Poon###

Half Heusler Alloys for Efficient Thermoelectric Power Conversion. Half-Heusler (HH) phases (space group F43m, Clb) are increasingly gaining
attention as promising thermoelectric materials in view of their thermal
stability, scalability, and environmental benignity as well as efficient power
output. Until recently, the verifiable dimensionless figure of merit (ZT) of HH
phases has remained moderate near 1, which limits the power conversion
efficiency of these materials. We report herein ZT~1.3 in n-type (Hf,Zr)NiSn
alloys near 850 K developed through elemental substitution and simultaneously
embedment of nanoparticles in the HH matrix, obtained by annealing the samples
close to their melting temperatures. Introduction of mass fluctuation and
scattering centers play a key role in the high ZT measured, as shown by the
reduction of thermal conductivity and increase of thermopower. Based on
computation, the power conversion efficiency of a n-p couple module based on
the new n-type (Hf,Zr,Ti)NiSn particles-in-matrix composite and recently
reported high-ZT p-type HH phases is expected to reach 13%, comparable to that
of state-of-the-art materials, but with the mentioned additional materials and
environmental attributes. Since the high efficiency is obtained without tuning
the microstructure of the Half-Heusler phases, it leaves room for further
optimization.

###Selective Enhancement in Phonon Scattering leads to High Thermoelectric Figure of Merit in ZnO -- Graphene Oxide Core-shell Nanohybrids|Soumya Biswas,Saurabh Singh,Shubham Singh,Shashwata Chattopadhyay,K. K. H. De Silva,M. Yoshimura,J. Mitra,Vinayak Kamble###

Selective Enhancement in Phonon Scattering leads to High Thermoelectric Figure of Merit in ZnO -- Graphene Oxide Core-shell Nanohybrids. ZnO is a promising candidate as an environment friendly thermoelectric (TE)
material. However, the poor TE figure of merit (zT) needs to be addressed to
achieve significant TE efficiency for commercial applications. Here we
demonstrate that selective enhancement in phonon scattering leads to increase
in zT of RGO encapsulated Al-doped ZnO core shell nanohybrids, synthesized via
a facile and scalable method. The incorporation of 1 at% Al with 1.5 wt% RGO
into ZnO (AGZO) has been found to show significant enhancement in zT (=0.52 at
1100 K) which is an order of magnitude larger compared to that of bare undoped
ZnO. Photoluminescence and X-ray photoelectron spectroscopy measurements
confirm that RGO encapsulation significantly quenches surface oxygen vacancies
in ZnO along with nucleation of new interstitial Zn donor states. Tunneling
spectroscopy reveals that the band gap of ~ 3.4 eV for bare ZnO reduces
effectively to ~ 0.5 eV upon RGO encapsulation, facilitating charge transport.
The electrical conductivity enhancement also benefits from the more than 95%
densification achieved, using the spark plasma sintering method, which aids
reduction of GO into RGO. The same Al doping and RGO capping synergistically
brings about drastic reduction of thermal conductivity, through enhanced
phonon-phonon and point defect-phonon scatterings. These opposing effects on
electrical and thermal conductivities enhances the power factors as well as the
zT value. Overall, a practically viable route for synthesis of oxide - RGO TE
material which could find its practical applications for the high-temperature
TE power generation.

###Thermoelectric transport trends in group 4 half-Heusler alloys|Kristian Berland,Nina Shulumba,Olle Hellman,Clas Persson,Ole Martin Løvvik###

Thermoelectric transport trends in group 4 half-Heusler alloys. The thermoelectric properties of 54 different group 4 half-Heusler (HH)
alloys have been studied from first principles. Electronic transport was
studied with density functional theory using hybrid functionals facilitated by
the $\mathbf{k} \cdot \mathbf{p}$ method, while the temperature dependent
effective potential method was used for the phonon contributions to the figure
of merit $ZT$. The phonon thermal conductivity was calculated including
anharmonic phonon-phonon, isotope, alloy and grain-boundary scattering. HH
alloys have an ${\it XYZ}$ composition and those studied here are in the group
4-9-15 (Ti,Zr,Hf)(Co,Rh,Ir)(As,Sb,Bi) and group 4-10-14
(Ti,Zr,Hf)(Ni,Pd,Pt)(Ge,Sn,Pb). The electronic part of the thermal conductivity
was found to significantly impact $ZT$ and thus the optimal doping level.
Furthermore, the choice of functional was found to significantly affect
thermoelectric properties, particularly for structures exhibiting band
alignment features. The intrinsic thermal conductivity was significantly
reduced when alloy and grain boundary scattering were accounted for, which also
reduced the spread in thermal conductivity. It was found that sub-lattice
disorder on the ${\it Z}$-site, i.e. the site occupied by group 14 or 15
elements, was more effective than ${\it X}$-site substitution, occupied by
group 4 elements. The calculations confirmed that ZrNiSn, ZrCoSb and ZrCoBi
based alloys display promising thermoelectric properties. A few other n-type
and p-type compounds were also predicted to be potentially excellent
thermoelectric materials, given that sufficiently high charge carrier
concentrations can be achieved. This study provides insight into the
thermoelectric potential of HH alloys and casts light on strategies to optimize
thermoelectric performance of multicomponent alloys.

###A numerical study on the design trade-offs of a thin-film thermoelectric generator for large-area applications|Kirsi Tappura###

A numerical study on the design trade-offs of a thin-film thermoelectric generator for large-area applications. Thin-film thermoelectric generators with a novel folding scheme are proposed
for large-area, low energy-density applications. Both the electrical current
and heat transfer are in the plane of the thermoelectric thin-film, yet the
heat transfer is across the plane of the module - similar to conventional bulk
thermoelectric modules. With such designs, the heat leakage through the module
itself can be minimized and the available temperature gradient maximized.
Different from the previously reported corrugated thermoelectric generators,
the proposed folding scheme enables high packing densities without compromising
the thermal contact area to the heat source and sink. The significance of
various thermal transport, or leakage, mechanisms in relation to power
production is demonstrated for different packing densities and thicknesses of
the module under heat sink-limited conditions. It is shown that the power
factor is more important than ZT for predicting the power output of such
thin-film devices. As very thin thermoelectric films are employed with modest
temperature gradients, high aspect-ratio elements are needed to meet the -
usually ignored - requirements of practical applications for the current. With
the design trade-offs considered, the proposed devices may enable the
exploitation of thermoelectric energy harvesting in new - large-area -
applications at reasonable cost.

###Prediction of the High Thermoelectric Performance of Pnictogen Dichalcogenide Layered Compounds with Quasi-One-Dimensional Gapped Dirac-like Band Dispersion|Masayuki Ochi,Hidetomo Usui,Kazuhiko Kuroki###

Prediction of the High Thermoelectric Performance of Pnictogen Dichalcogenide Layered Compounds with Quasi-One-Dimensional Gapped Dirac-like Band Dispersion. Thermoelectric power generation has been recognized as one of the most
important technologies, and high-performance thermoelectric materials have long
been pursued. However, because of the large number of candidate materials, this
quest is extremely challenging, and it has become clear that a firm theoretical
concept from the viewpoint of band-structure engineering is needed. In this
study, we theoretically demonstrate that pnictogen-dichalcogenide layered
compounds, which originally attracted attention as a family of superconductors
and have recently been investigated as thermoelectric materials, can exhibit
very high thermoelectric performance with elemental substitution. In
particular, we clarify a promising guiding principle for materials design and
find that LaOAsSe$_2$, a material that has yet to be synthesized, has a
powerfactor that is six times as large as that of the known compound LaOBiS$_2$
and can exhibit a very large $ZT$ under some plausible assumptions. This large
enhancement of the thermoelectric performance originates from the
quasi-one-dimensional gapped Dirac-like band dispersion, which is realized by
the square-lattice network. Our study offers one ideal limit of the band
structure for thermoelectric materials. Because our target materials have high
controllability of constituent elements and feasibility of carrier doping,
experimental studies along this line are strongly awaited.

###Edge currents and nanopore arrays in zigzag and chiral graphene nanoribbons as a route toward high-$ZT$ thermoelectrics|Po-Hao Chang,Branislav K. Nikolic###

Edge currents and nanopore arrays in zigzag and chiral graphene nanoribbons as a route toward high-$ZT$ thermoelectrics. We analyze electronic and phononic quantum transport through zigzag or chiral
graphene nanoribbons (GNRs) perforated with an array of nanopores. Since local
charge current profiles in these GNRs are peaked around their edges, drilling
nanopores in their interior does not affect such edge charge currents while
drastically reducing heat current carried by phonons in sufficiently long
wires. The combination of these two effects can yield highly efficient
thermoelectric devices with maximum $ZT \simeq 11$ at liquid nitrogen
temperature and $ZT \simeq 4$ at room temperature achieved in $\sim 1$ $\mu$m
long zigzag GNRs with nanopores of variable diameter and spacing between them.
Our analysis is based on the $\pi$-orbital tight-binding Hamiltonian with up to
third nearest-neighbor hopping for electronic subsystem, the empirical
fourth-nearest-neighbor model for phononic subsystem, and nonequilibrium Green
function formalism to study quantum transport in both of these models.

###Thermoelectric properties of Pr3Rh4Sn13-type Yb3Co4Ge13 and Yb3Co4Sn13 compounds|A. V. Morozkin,V. Yu. Irkhin,V. N. Nikiforov###

Thermoelectric properties of Pr3Rh4Sn13-type Yb3Co4Ge13 and Yb3Co4Sn13 compounds. Crystallographic data and thermoelectric properties (from 240 K up to 380 K)
of Yb3Co4Ge13, Yb3Co4Sn13 compounds and Yb2CeCo4Ge13 and Yb2.3La0.7Co4Ge13
solid solutions are reported. The Seebeck coefficients, electric resistance and
thermal conductivity increase monotonically with increasing temperature from
240 to 380 K for all the compounds. The Seebeck coefficient is S = 14 - 27 \mu
V//K for Yb3Co4Sn13, and S = -21 -- -12 \mu V/K for Yb3Co4Ge13. The
substitution of Yb for cerium or lanthanum in Yb3Co4Ge13 shifts the Seeebeck
coefficient to positive values. The Yb3Co4Sn13 has a maximal ZT parameter from
available Pr3Rh4Sn13-type compounds. The ZT parameter of Yb3Co4Sn13 compound
increases from 0.006 up to 0.017 with increasing temperature from 240 K to 380
K.

###High $n$-type thermoelectric power factor and efficiency in Ba$_{2}$BiAu from a highly dispersive band|Junsoo Park,Yi Xia,Vidvuds Ozoliņš###

High $n$-type thermoelectric power factor and efficiency in Ba$_{2}$BiAu from a highly dispersive band. Using first-principles density-functional theory calculations, we predict the
potential for unprecedented thermoelectric efficiency $zT=5$ at 800 K in
$n$-type Ba$_{2}$BiAu full-Heusler compound. Such a high efficiency arises from
an intrinsically ultralow lattice thermal conductivity coupled with a very high
power factor reaching 7 mW m$^{-1}$ K$^{-2}$ at 500 K. The high power factor
originates from a light, sixfold degenerate conduction band pocket along the
$\Gamma$-X direction. Weak acoustic phonon scattering and sixfold multiplicity
combine to yield high mobility and high Seebeck coefficient. In contrast, the
flat-and-dispersive (a.k.a. low-dimensional) valence band of Ba$_{2}$BiAu fail
to generate a high power factor due to strong acoustic phonon scattering. The
Lorenz numbers at optimal doping are smaller than the Wiedemann-Franz value, an
integral feature for $zT$ enhancement as electrons are the majority heat
carriers.

###Even-odd effect in the thermopower and strongly enhanced thermoelectric efficiency for superconducting single-electron transistors|Christopher Eltschka,Jens Siewert###

Even-odd effect in the thermopower and strongly enhanced thermoelectric efficiency for superconducting single-electron transistors. It is well known that the transport properties of single-electron transistors
with a superconducting island and normal-conducting leads (NSN SET) may depend
on whether or not there is a single quasiparticle on the island. This parity
effect has pronounced consequences for the linear transport properties. Here we
analyze the thermopower of NSN SET with and without parity effect, for entirely
realistic values of device parameters. Besides a marked dependence of the
thermopower on the superconducting gap $\Delta$ we observe an enhancement in
the parity regime which is accompanied by a dramatic increase of the
thermoelectric figure of merit ZT. The latter can be explained within a simple
re-interpretation of ZT in terms of averages and variances of transport
energies.

###Transport property analysis method for thermoelectric materials: material quality factor and the effective mass model|Stephen Dongmin Kang,G. Jeffrey Snyder###

Transport property analysis method for thermoelectric materials: material quality factor and the effective mass model. Thermoelectric semiconducting materials are often evaluated by their
figure-of-merit, zT. However, by using zT as the metric for showing
improvements, it is not immediately clear whether the improvement is from an
enhancement of the inherent material property or from optimization of the
carrier concentration. Here, we review the quality factor approach which allows
one to separate these two contributions even without Hall measurements. We
introduce practical methods that can be used without numerical integration. We
discuss the underlying effective mass model behind this method and show how it
can be further advanced to study complex band structures using the Seebeck
effective mass. We thereby dispel the common misconception that the usefulness
of effective band models is limited to single parabolic band materials.

###Thermoelectric conversion at 30K in InAs/InP nanowire quantum dots|Domenic Prete,Paolo Andrea Erdman,Valeria Demontis,Valentina Zannier,Daniele Ercolani,Lucia Sorba,Fabio Beltram,Francesco Rossella,Fabio Taddei,Stefano Roddaro###

Thermoelectric conversion at 30K in InAs/InP nanowire quantum dots. We demonstrate high-temperature thermoelectric conversion in InAs/InP
nanowire quantum dots by taking advantage of their strong electronic
confinement. The electrical conductance G and the thermopower S are obtained
from charge transport measurements and accurately reproduced with a theoretical
model accounting for the multi-level structure of the quantum dot. Notably, our
analysis does not rely on the estimate of co-tunnelling contributions since
electronic thermal transport is dominated by multi-level heat transport. By
taking into account two spin-degenerate energy levels we are able to evaluate
the electronic thermal conductance K and investigate the evolution of the
electronic figure of merit ZT as a function of the quantum dot configuration
and demonstrate ZT ~ 35 at 30 K, corresponding to an electronic effciency at
maximum power close to the Curzon- Ahlborn limit.

###High thermoelectric performance of two-dimensional (PbTe)2 layer|Caiyu Sheng,Dengdong Fan,Huijun Liu###

High thermoelectric performance of two-dimensional (PbTe)2 layer. The electronic, phonon and thermoelectric transport properties of (PbTe)2
layer are systematically investigated by using first-principles pseudopotential
method and Boltzmann transport equation. Our calculations demonstrate that
there is a valley degeneracy of six for the top valence band, which leads to
larger carrier concentration and thus higher electrical conductivity without
obvious reduction in the Seebeck coefficient. Moreover, the intrinsic van der
Waals interactions between neighboring Pb layers induce additional phonon
scattering and thus ultrasmall lattice thermal conductivity. As a consequence,
a maximum p-type ZT value of 2.9 can be achieved at 1000 K. Moreover, we find
almost identical n- and p-type ZT in the temperature range from 300 K to 800 K.

###On the structure and electronic properties of Fe$_2$V$_{0.8}$W$_{0.2}$Al thin films|E. Alleno,A. Berche,J. -C. Crivello,A. Diack Rasselio,P. Jund###

On the structure and electronic properties of Fe$_2$V$_{0.8}$W$_{0.2}$Al thin films. A very large thermoelectric figure of merit ZT = 6 at 380 K has recently been
reported in Fe$_2$V$_{0.8}$W$_{0.2}$Al under thin-film form (Hinterleitner et
al., Nature 576 (2019) 85). Under this form, Fe$_2$V$_{0.8}$W$_{0.2}$Al
experimentally crystallizes in a disordered A2 crystal structure, different
from its bulk-form structure (L21). First principles calculations of the
electronic structure performed in A2-Fe$_2$V$_{0.8}$W$_{0.2}$Al supercells
generated by the Special Quasi-random Structure (SQS) method are thus reported
here. These calculations unambiguously indicate that
A2-Fe$_2$V$_{0.8}$W$_{0.2}$Al is a ferromagnetic metal at 0 K, displaying a
small Seebeck coefficient at 400 K (< 30 microV/K). The present results
contradict the scenario of the occurrence of a deep pseudo-gap at the Fermi
level, previously invoked to justify ZT = 6 in Fe$_2$V$_{0.8}$W$_{0.2}$Al thin
films.

###Thermal conductivity and enhanced thermoelectric performance of SnTe bilayer|Abhiyan Pandit,Raad Haleoot,Bothina Hamad###

Thermal conductivity and enhanced thermoelectric performance of SnTe bilayer. Tin chalcogenides (SnS, SnSe, and SnTe) are found to have improved
thermoelectric properties upon the reduction of their dimensionality. Here we
found the tilted AA + s stacked two-dimensional (2D) SnTe bilayer as the most
stable phase among several stackings as predicted by the structural
optimization and phonon transport properties. The carrier mobility and
relaxation time are evaluated using the deformation potential theory, which is
found to be relatively high due to the high 2D elastic modulus, low deformation
potential constant, and moderate effective masses. The SnTe bilayer shows a
high Seebeck coefficient, high electrical conductivity, and ultralow lattice
thermal conductivity. High TE figure of merit (ZT) values, as high as 4.61
along the zigzag direction, are predicted for the SnTe bilayer. These ZT values
are much enhanced as compared to the bulk as well as monolayer SnTe and other
2D compounds.

###A micrometer-thick oxide film with high thermoelectric performance at temperature ranging from 20-400 K|Jikun Chen,Hongyi Chen,Feng Hao,Xinyou Ke,Nuofu Chen,Takeaki Yajima,Yong Jiang,Xun Shi,Kexiong Zhou,Max Döbeli,Tiansong Zhang,Binghui Ge,Hongliang Dong,Huarong Zeng Wenwang Wu,Lidong Chen###

A micrometer-thick oxide film with high thermoelectric performance at temperature ranging from 20-400 K. Thermoelectric (TE) materials achieve localised conversion between thermal
and electric energies, and the conversion efficiency is determined by a figure
of merit zT. Up to date, two-dimensional electron gas (2DEG) related TE
materials hold the records for zT near room-temperature. A sharp increase in zT
up to ~2.0 was observed previously for superlattice materials such as PbSeTe,
Bi2Te3/Sb2Te3 and SrNb0.2Ti0.8O3/SrTiO3, when the thicknesses of these TE
materials were spatially confine within sub-nanometre scale. The
two-dimensional confinement of carriers enlarges the density of states near the
Fermi energy3-6 and triggers electron phonon coupling. This overcomes the
conventional {\sigma}-S trade-off to more independently improve S, and thereby
further increases thermoelectric power factors (PF=S2{\sigma}). Nevertheless,
practical applications of the present 2DEG materials for high power energy
conversions are impeded by the prerequisite of spatial confinement, as the
amount of TE material is insufficient. Here, we report similar TE properties to
2DEGs but achieved in SrNb0.2Ti0.8O3 films with thickness within sub-micrometer
scale by regulating interfacial and lattice polarizations. High power factor
(up to 103 {\mu}Wcm-1K-2) and zT value (up to 1.6) were observed for the film
materials near room-temperature and below. Even reckon in the thickness of the
substrate, an integrated power factor of both film and substrate approaching to
be 102 {\mu}Wcm-1K-2 was achieved in a 2 {\mu}m-thick SrNb0.2Ti0.8O3 film grown
on a 100 {\mu}m-thick SrTiO3 substrate. The dependence of high TE performances
on size-confinement is reduced by ~103 compared to the conventional
2DEG-related TE materials. As-grown oxide films are less toxic and not
dependent on large amounts of heavy elements, potentially paving the way
towards applications in localised refrigeration and electric power generations.

###Large Thermoelectric Power Factor in TiS2 Crystal with Nearly Stoichiometric Composition|H. Imai,Y. Shimakawa,Y. Kubo###

Large Thermoelectric Power Factor in TiS2 Crystal with Nearly Stoichiometric Composition. A TiS$_{2}$ crystal with a layered structure was found to have a large
thermoelectric power factor.The in-plane power factor $S^{2}/ \rho$ at 300 K is
37.1~$\mu$W/K$^{2}$cm with resistivity ($\rho$) of 1.7 m$\Omega$cm and
thermopower ($S$) of -251~$\mu$V/K, and this value is comparable to that of the
best thermoelectric material, Bi$_{2}$Te$_{3}$ alloy. The electrical
resistivity shows both metallic and highly anisotropic behaviors, suggesting
that the electronic structure of this TiS$_{2}$ crystal has a
quasi-two-dimensional nature. The large thermoelectric response can be ascribed
to the large density of state just above the Fermi energy and inter-valley
scattering. In spite of the large power factor, the figure of merit, $ZT$ of
TiS$_{2}$ is 0.16 at 300 K, because of relatively large thermal conductivity,
68~mW/Kcm. However, most of this value comes from reducible lattice
contribution. Thus, $ZT$ can be improved by reducing lattice thermal
conductivity, e.g., by introducing a rattling unit into the inter-layer sites.

###Do Thermoelectric Materials in Nanojunctions Display Material Property or Junction Property?|Yu-Chang Chen,Yu-Shen Liu###

Do Thermoelectric Materials in Nanojunctions Display Material Property or Junction Property?. The miniaturization of thermoelectric nanojunctions raises a fundamental
question: do the thermoelectric quantities of the bridging materials in
nanojunctions remain to display material properties or show junction
properties? In order to answer this question, we investigate the Seebeck
coefficient $S$ and the thermoelectric figure of merit $ZT$ especially in
relation to the length characteristics of the junctions from the
first-principles approaches. For $S$, the metallic atomic chains reveal strong
length characteristics related to strong hybridization in the electronic
structures between the atoms and electrodes, while the insulating molecular
wires display strong material properties due to the cancelation of exponential
scalings in the DOSs. For $ZT$, the atomic wires remain to show strong junction
properties. However, the length chrematistics of the insulation molecular wires
depend on a characteristic temperature $T_{0}= \sqrt{\beta/\gamma(l)}$ around
10K. When $T \ll T_{0}$, where the electron transport dominates the thermal
current, the molecular junctions remain to show material properties. When $T
\gg T_{0}$, where the phonon transport dominates the thermal current, the
molecular junctions display junction properties.

###Thermoelectric properties of electrically gated bismuth telluride nanowires|Igor Bejenari,Valeriu Kantser,Alexander A. Balandin###

Thermoelectric properties of electrically gated bismuth telluride nanowires. We theoretically studied the effect of the perpendicular electric field on
the thermoelectric properties of the intrinsic, n-type and p-type bismuth
telluride nanowires with the growth direction [110]. The electronic structure
and the wave functions were calculated by solving self-consistently the system
of the Schrodinger and Poisson equations using the spectral method. The Poisson
equation was solved in terms of the Newton - Raphson method within the
predictor-corrector approach. The electron - electron exchange - correlation
interactions were taken into account in our analysis. In the temperature range
from 77 to 500 K, the dependences of the Seebeck coefficient, thermal
conductivity, electron (hole) concentration, and thermoelectric figure of merit
on the nanowire thickness, gate voltage, and excess hole (electron)
concentration were investigated in the constant relaxation-time approximation.
The results of our calculations indicate that the external perpendicular
electric field can increase the Seebeck coefficient of the bismuth telluride
nanowires with thicknesses of 7 - 15 nm by nearly a factor of 2 and enhance ZT
by an order of magnitude. At room temperature, ZT can reach a value as high as
3.4 under the action of the external perpendicular electric field for realistic
widths of the nanowires. The obtain results may open up a completely new way
for a drastic enhancement of the thermoelectric figure of merit in a wide
temperature range.

###Thermoelectric transport in strained Si and Si/Ge heterostructures|N. F. Hinsche,I. Mertig,P. Zahn###

Thermoelectric transport in strained Si and Si/Ge heterostructures. The anisotropic thermoelectric transport properties of bulk silicon strained
in [111]-direction were studied by detailed first-principles calculations
focussing on a possible enhancement of the power factor. Electron as well as
hole doping were examined in a broad doping and temperature range. At low
temperature and low doping an enhancement of the power factor was obtained for
compressive and tensile strain in the electron-doped case and for compressive
strain in the hole-doped case. For the thermoelectrically more important high
temperature and high doping regime a slight enhancement of the power factor was
only found under small compressive strain with the power factor overall being
robust against applied strain. To extend our findings the anisotropic
thermoelectric transport of an [111]-oriented Si/Ge superlattice was
investigated. Here, the cross-plane power factor under hole-doping was
drastically suppressed due to quantum-well effects, while under electron-doping
an enhanced power factor was found. With that, we state a figure of merit of
ZT$=0.2$ and ZT$=1.4$ at $T=\unit[300]{K}$ and $T=\unit[900]{K}$ for the
electron-doped [111]-oriented Si/Ge superlattice. All results are discussed in
terms of band structure features.

###Enhanced thermoelectric performance in Ca substituted Sr3SnO|Enamul Haque,M. Anwar Hossain###

Enhanced thermoelectric performance in Ca substituted Sr3SnO. We report 45% enhancement in the thermoelectric figure of merit, ZT of Sr3SnO
via Ca substitution. First-principles calculations have been performed to study
the electronic and thermoelectric transport properties of Ca substituted Sr3SnO
(Sr3-xCaxSnO). The effects of Ca subtitution on bandgap are studied and
detailed mechanisms are proposed to explain the obtained results. We have found
that effective mass and thermopower of Sr3SnO redueces with the increase of
hole concentration. The optimum hole concentration has been obtained for
Sr2CaSnO and the corresponding Seebeck coefficient is 219 {\mu}V/K. The
electrical conductivity of Sr3SnO and its alloys exhibits semiconducting nature
which contradicts with experimental results in Ca3SnO. We have found that due
to the Ca-deficiency, the Ca3SnO shows the metallic conductivity and removes
this contradiction with our results. The lattice thermal conductivities (\kl)
of Sr3SnO and Ca3SnO have been calculated by using both PBE and GW functionals.
The lattice thermal conductivity obtained by PBE functional largely
underestimates the experimental value for Ca3SnO. The total thermal
conductivity (with kl obtained by GW) at 300K is 2.33 and 1.897 W/mK for Sr3SnO
and Ca3SnO, respectively, with excellent agreement with experimental value
1.707 W/mK for Ca3SnO. The dimensionless figure of merit (ZT) for Sr2CaSnO at
500 K is 0.6 and making it promising for thermoelectric applications.

###Extreme Low Thermal Conductivity in Nanoscale 3D Si Phononic Crystal with Spherical Pores|Lina Yang,Nuo Yang,Baowen Li###

Extreme Low Thermal Conductivity in Nanoscale 3D Si Phononic Crystal with Spherical Pores. Thermoelectric material provides a high hope for converting harmful and
useless heat into useful energy, electricity. It can also be used as solid
state Peltier coolers in integrated circuits, an outstanding challenge for
electronic engineers. The desire for a high efficient thermoelectric material
has never been so keen. Although many works have been done, we are still far
from having a recipe for thermoelectric materials. Nano-structuring provides an
effective way to increase figure of merit (ZT) by reducing the thermal
conductivity without affecting electronic property.1 Here, we propose a novel
nanoscale three-dimensional (3D) Si phononic crystal (PnC) with spherical
pores, which can reduce the thermal conductivity of bulk Si by a factor up to
10,000 times at room temperature. The extreme-low thermal conductivity could
lead to a larger value of ZT than unity. The thermal conductivity changes
little when temperature increases from room temperature to 1100 K. The phonon
participation ratio spectra show there are more phonon localizations as the
porosity of PnC increases.

###Potential thermoelectric material $\mathrm{Cs_2[PdCl_4]I_2}$: a first-principles study|San-Dong Guo###

Potential thermoelectric material $\mathrm{Cs_2[PdCl_4]I_2}$: a first-principles study. The electronic structures and thermoelectric properties of
$\mathrm{Cs_2[PdCl_4]I_2}$ are investigated by the first-principles
calculations and semiclassical Boltzmann transport theory. Both electron and
phonon transport are considered to attain the figure of merit $ZT$. A modified
Becke and Johnson (mBJ) exchange potential, including spin-orbit coupling
(SOC), is employed to investigate electronic part of
$\mathrm{Cs_2[PdCl_4]I_2}$. It is found that SOC has obvious effect on valence
bands, producing huge spin-orbital splitting, which leads to remarkable
detrimental effect on p-type power factor. However, SOC has a negligible
influence on conduction bands, so the n-type power factor hardly change. The
temperature dependence of lattice thermal conductivity by assuming an inverse
temperature dependence is attained from reported ultralow lattice thermal
conductivity of 0.31 $\mathrm{W m^{-1} K^{-1}}$ at room temperature.
Calculating scattering time $\tau$ is challenging, but a hypothetical $\tau$
can be adopted to estimate thermoelectric conversion efficiency. The maximal
figure of merit $ZT$ is up to about 0.70 and 0.60 with scattering time
$\tau$=$10^{-14}$ s and $\tau$=$10^{-15}$ s, respectively. These results make
us believe that $\mathrm{Cs_2[PdCl_4]I_2}$ may be a potential thermoelectric
material.

###Thermoelectric properties of half-Heusler $\mathrm{ZrNiPb}$ by using first principles calculations|San-Dong Guo###

Thermoelectric properties of half-Heusler $\mathrm{ZrNiPb}$ by using first principles calculations. We investigate electronic structures and thermoelectric properties of recent
synthetic half-Heusler $\mathrm{ZrNiPb}$ by using generalized gradient
approximation (GGA) and GGA plus spin-orbit coupling (GGA+SOC). Calculated
results show that $\mathrm{ZrNiPb}$ is a indirect-gap semiconductor. Within the
constant scattering time approximation, semi-classic transport coefficients are
performed through solving Boltzmann transport equations. It is found that the
SOC has more obvious influence on power factor in p-type doping than in n-type
doping, leading to a detrimental effect in p-type doping. These can be
explained by considering the SOC influences on the valence bands and conduction
bands near the Fermi level. The lattice thermal conductivity as a function of
temperature is calculated, and the corresponding lattice thermal conductivity
is 14.5 $\mathrm{W m^{-1} K^{-1}}$ at room temperature. By comparing the
experimental transport coefficients with calculated ones, the scattering time
is attained for 0.333 $\times$ $10^{-14}$ s. Finally, the thermoelectric figure
of merit $ZT$ can be attained, and the $ZT$ value can be as high as 0.30 at
high temperature by choosing appropriate doping level. It is possible to reduce
lattice thermal conductivity by point defects and boundaries, and make
half-Heusler $\mathrm{ZrNiPb}$ become potential candidate for efficient
thermoelectricity.

###Theoretical Investigation on the Effect of multinary Isoelectronic Substitution on TiCoSb based half-Heusler alloys|Mukesh K. Choudhary,P. Ravindran###

Theoretical Investigation on the Effect of multinary Isoelectronic Substitution on TiCoSb based half-Heusler alloys. To understand the effect of isoelectronic substitution on thermoelectric
properties of TiCoSb based half - Heusler (HH) alloys, we have systematically
studied the transport properties with substitution of Zr at Ti and Bi at Sb
sites. The electronic structure of TixZr1-xCoSbxBi1-x (x = 0.25, 0.5, 0.75) and
parent TiCoSb are investigated using the full potential linearized augmented
plane wave method and the thermoelectric transport properties are calculated on
the basis of semiclassical Boltzmann transport theory. The band analysis of the
calculated band structures reveal that TixZr1-xCoSbxBi1-x has semiconducting
behavior with indirect band gap at x = 0.25, 0.5 concentration and direct band
gap behavior at x = 0.75 concentration. The TixZr1-xCoSbxBi1-x (x = 0.25, 0.5,
0.75) compounds show smaller band gap values as compared to the pure TiCoSb.
The d electrons of Ti/Zr and Co dominate the electronic transport properties of
TixZr1-xCoSbxBi1-x system. All these systems follow the empirical rule of 18
valence-electron content to bring semiconductivity in HH alloys. The
isoelectronic substitution in TiCoSb can tune the band structure by shifting
the Fermi level. This provides us lot of possibilities to get the desired band
gap values for designing thermoelectrics with high efficiency. In this study we
have showed that the isoelectronic substitution at both Ti and Sb site of
TiCoSb has very small effect for increasing the ZT values and one should go for
isoelectronic substitution at any one sites of TiCoSb HH alloys alone to
improve ZT.

###Effect of multinary substitution on electronic and transport properties of TiCoSb based half-Heusler alloys|Mukesh K. Choudhary,P. Ravindran###

Effect of multinary substitution on electronic and transport properties of TiCoSb based half-Heusler alloys. The electronic structures of TixZrx/2CoPbxTex, TixZrx/2Hfx/2CoPbxTex (x =
0.5), and the parent compound TiCoSb were investigated using the full potential
linearized augmented plane wave method. The thermoelectric transport properties
of these alloys are calculated on the basis of semi-classical Boltzmann
transport theory. From the band structure calculations we show that the
substitution of Zr,Hf in the Ti site and Pb and Te in the Sb site lower the
band gap value and also change the indirect band (IB) gap of TiCoSb to the
direct band (DB) gap. The calculated band gap of TiCoSb, TixZrx/2CoPbxTex, and
TixZrx/2Hfx/2CoPbxTex are 1.04 eV (IB), 0.92 eV (DB), and 0.93 eV (DB),
respectively. All these alloys follow the empirical rule of 18 valence-electron
content which is essential for bringing semiconductivity in half Heusler
alloys. It is shown that the substitution of Hf at the Ti site improve the ZT
value (~1.05) at room temperature, whereas there is no significant difference
in ZT is found at higher temperature. Based on the calculated thermoelectric
transport properties, we conclude that the appropriate concentration of Hf
substitution can further improve the thermoelectric performance of
TixZrx/2Hfx/2CoPbxTex.

###High thermoelectric performance in the hexagonal bilayer structure consisting of light boron and phosphorus elements|Z. Z. Zhou,H. J. Liu,D. D. Fan,G. H. Cao,C. Y. Sheng###

High thermoelectric performance in the hexagonal bilayer structure consisting of light boron and phosphorus elements. Two-dimensional layered materials have attracted tremendous attentions due to
their extraordinary physical and chemical properties. Using first-principles
calculations and Boltzmann transport theory, we give an accurate prediction of
the thermoelectric properties of boron phosphide (BP) bilayer, where the
carrier relaxation time is treated within the framework of electron-phonon
coupling. It is found that the lattice thermal conductivity of BP bilayer is
much lower than that of its monolayer structure, which can be attributed to the
presence of van der Waals interactions. On the other hand, the graphene-like BP
bilayer shows very high carrier mobility with a moderate band gap of 0.88 eV.
As a consequence, a maximum p-type ZT value of ~1.8 can be realized along the
x-direction at 1200 K, which is amazingly high for systems consisting of light
elements only. Moreover, we obtain almost identical p- and n-type ZT of ~1.6
along the y-direction, which is very desirable for fabrication of
thermoelectric modules with comparative efficiencies. Collectively, these
findings demonstrate great advantages of the layered structures containing
earth-abundant elements for environment-friendly thermoelectric applications.

###Unusual thermoelectric transport anisotropy in quasi-2D, rhombohedral GeTe|Vahid Askarpour,Jesse Maassen###

Unusual thermoelectric transport anisotropy in quasi-2D, rhombohedral GeTe. In this study, we calculate the $T$=300 K scattering and thermoelectric
transport properties of rhombohedral GeTe using first-principles modeling. The
room-temperature phase of GeTe has a layered structure, with cross-plane and
in-plane directions oriented parallel and perpendicular to [111], respectively.
Based on rigorous electron-phonon scattering, our transport calculations reveal
unusual anisotropic properties; n-type GeTe has a cross-plane electrical
conductivity that is roughly 3$\times$ larger than in-plane. p-type GeTe,
however, displays opposite anisotropy with in-plane conducting roughly
2$\times$ more than cross-plane, as is expected in quasi-2D materials. The
power factor shows the same anisotropy as the electrical conductivity, since
the Seebeck coefficient is relatively isotropic. Interestingly, cross-plane
n-GeTe shows the largest mobility and power factor approaching 500 cm$^2$/V-s
and 32 $\mu$W/cm-K$^2$, respectively. The thermoelectric figure-of-merit, $zT$,
is enhanced as a result of this unusual anisotropy in n-GeTe since the lattice
thermal conductivity is minimized along cross-plane. This decouples the
preferred transport directions of electrons and phonons, leading to a threefold
increase in $zT$ along cross-plane compared to in-plane. The n-type anisotropy
results from high-velocity electron states formed by Ge p-orbitals that span
across the interstitial region. This surprising behavior, that would allow the
preferential conduction direction to be controlled by doping, could be observed
in other quasi-2D materials and exploited to achieve higher-performance
thermoelectrics.

###First principles study on the thermoelectric performance of CaAl$_2$Si$_2$-type Zintl phase compounds|Hidetomo Usui,Kazuhiko Kuroki###

First principles study on the thermoelectric performance of CaAl$_2$Si$_2$-type Zintl phase compounds. We investigate the thermoelectric properties of CaAl$_2$Si$_2$-type Zintl
phase compounds $AB_2X_2$ ($A$ = Mg, Ca, Sr, Ba, $B$ = Mg, Zn, Cd, and $X$ = P,
As, Sb) using first principles band calculations within the Boltzmann transport
theory assuming the constant relaxation time approximation. We introduce the
effective degree of valley degeneracy $n_{TE}$ to focus on the relationship
between the thermoelectric properties and the multivalley character of the
electronic band structure around the Fermi level. We also introduce a quantity
$\gamma_{TE}$, which takes into account $n_{TE}$ and anisotropy of the valley
structure, and it is found that $\gamma_{TE}$ enables us to well understand the
overall trend of the material dependence of the power factor. We finally
suggest promising thermoelectric materials, e.g. BaMg$_2$P$_2$ for PF $\sim
20\mu$W/cmK$^2$ and $ZT > 0.2$ at 300K and SrZn$_2$As$_2$ for PF $\sim
35\mu$W/cmK$^2$ and $ZT > 0.35$ at 300K assuming a relaxation time of 10 fs and
a lattice thermal conductivity value of 2 W/mK.

###Probing efficient n-type lanthanide dopants for Mg$_3$Sb$_2$ thermoelectrics|Jiawei Zhang,Lirong Song,Bo Brummerstedt Iversen###

Probing efficient n-type lanthanide dopants for Mg$_3$Sb$_2$ thermoelectrics. The recent discovery of n-type Mg$_3$Sb$_2$ thermoelectric has ignited
intensive research activities on searching for potential n-type dopants for
this material. Using first-principles defect calculations, here we conduct a
systematic computational screening of potential efficient n-type lanthanide
dopants for Mg$_3$Sb$_2$. In addition to La, Ce, Pr, and Tm, we find that high
electron concentration ($\geq$ 10$^{20}$ cm$^{-3}$ at the growth temperature of
900 K) can be achieved by doping on the Mg sites with Nd, Gd, Ho, and Lu, which
are generally more efficient than other lanthanide dopants and the anion-site
dopant Te. Experimentally, we confirm Nd and Tm as effective n-type dopants for
Mg$_3$Sb$_2$ since doping with Nd and Tm shows superior thermoelectric figure
of merit zT $\geq$ 1.3 with higher electron concentration than doping with Te.
Through codoping with Nd (Tm) and Te, simultaneous power factor improvement and
thermal conductivity reduction are achieved. As a result, we obtain high zT
values of about 1.65 and 1.75 at 775 K in n-type
Mg$_{3.5}$Nd$_{0.04}$Sb$_{1.97}$Te$_{0.03}$ and
Mg$_{3.5}$Tm$_{0.03}$Sb$_{1.97}$Te$_{0.03}$, respectively, which are among the
highest values for n-type Mg$_3$Sb$_2$ without alloying with Mg$_3$Bi$_2$. This
work sheds light on exploring promising n-type dopants for the design of
Mg$_3$Sb$_2$ thermoelectrics.

###Optimal Band Structure for Thermoelectrics with Realistic Scattering and Bands|Junsoo Park,Yi Xia,Vidvuds Ozoliņš,Anubhav Jain###

Optimal Band Structure for Thermoelectrics with Realistic Scattering and Bands. Understanding how to optimize electronic band structures for thermoelectrics
is a topic of long-standing interest in the community. Prior models have been
limited to simplified bands and/or scattering models. In this study, we apply
more rigorous scattering treatments to more realistic model band structures -
upward-parabolic bands that inflect to an inverted parabolic behavior -
including cases of multiple bands. In contrast to common descriptors (e.g.,
quality factor and complexity factor), the degree to which multiple pockets
improve thermoelectric performance is bounded by interband scattering and the
relative shapes of the bands. We establish that extremely anisotropic
`flat-and-dispersive' bands, although best-performing in theory, may not
represent a promising design strategy in practice. Critically, we determine
optimum bandwidth, dependent on temperature and lattice thermal conductivity,
from perfect transport cutoffs that can in theory significantly boost $zT$
beyond the values attainable through intrinsic band structures alone. Our
analysis should be widely useful as the thermoelectric research community eyes
$zT>3$.

###First-principles study of anisotropic thermoelectric transport properties of IV-VI semiconductor compounds SnSe and SnS|Ruiqiang Guo,Xinjiang Wang,Youdi Kuang,Baoling Huang###

First-principles study of anisotropic thermoelectric transport properties of IV-VI semiconductor compounds SnSe and SnS. We conduct comprehensive investigations of both thermal and electrical
transport properties of SnSe and SnS using first-principles calculations
combined with the Boltzmann transport theory. Due to the distinct layered
lattice structure, SnSe and SnS exhibit similarly anisotropic thermal and
electrical behaviors. The cross-plane lattice thermal conductivity $\kappa_{L}$
is 40-60% lower than the in-plane values. Extremely low $\kappa_{L}$ is found
for both materials because of high anharmonicity. It is suggested that
nanostructuring would be difficult to further decrease $\kappa_{L}$ because of
the short mean free paths of dominant phonon modes (1-30 nm at 300 K) while
alloying would be efficient in reducing $\kappa_{L}$ considering that the
relative $\kappa_{L}$ contribution ($\sim$ 65%) of optical phonons is
remarkably large. On the electrical side, the anisotropic electrical
conductivities are mainly due to the different effective masses of holes and
electrons along the $a$, $b$ and $c$ axes. This leads to the highest optimal
$ZT$ values along the $b$ axis and lowest ones along the $a$ axis in both
$p$-type materials. However, the $n$-type ones exhibit the highest $ZT$s along
the $a$ axis due to the enhancement of power factor when the chemical potential
gradually approaches the secondary band valley that causes significant increase
in electron mobility and density of states. SnSe exhibits larger optimal $ZT$s
compared with SnS in both $p$-type and $n$-type materials. For both materials,
the peak $ZT$s of $n$-type materials are much higher than those of $p$-type
ones along the same direction. The predicted highest $ZT$ values at 750 K are
1.0 in SnSe and 0.6 in SnS along the $b$ axis for the $p$-type doping while
those for the $n$-type doping reach 2.7 in SnSe and 1.5 in SnS along the $a$
axis, rendering them among the best bulk thermoelectric materials for
large-scale applications.

###Decouple Electronic and Phononic Transport in Nanotwinned Structure: A New Strategy for Enhancing the Figure-of-merit of Thermoelectrics|Yanguang Zhou,Xiaojing Gong,Ben Xu,Ming Hu###

Decouple Electronic and Phononic Transport in Nanotwinned Structure: A New Strategy for Enhancing the Figure-of-merit of Thermoelectrics. Thermoelectrics (TE) materials manifest themselves in direct conversion of
temperature differences to electric power and vice versa. Despite remarkable
advances have been achieved in the past decades for various TE systems, the
energy conversion efficiencies of TE devices, which is characterized by a
dimensionless figure-of-merit (ZT ), remain a generally poor factor that
severely limits their competitiveness and range of employment. The bottleneck
for substantially boosting ZT coefficient lies in the strong interdependence of
the physical parameters involved in electronic and phononic transport. Here, we
propose a new strategy of incorporating nanotwinned structures to decouple the
electronic and phononic transport. Combining the new concept of nanotwin with
the previously widely used nanocrystalline approach, the power factor of the Si
nanotwin-nanocrystalline heterostructures is enhanced by 120% compared to bulk
crystalline Si, while the lattice thermal conductivity is reduced to a level
well below the amorphous limit, yielding a theoretical limit of 0.43 for ZT
coefficient at room temperature. This value is almost two orders of magnitude
larger than that for bulk Si and twice of the polycrystalline Si. Even for the
experimentally existing nanotwin-nanocrystalline heterostructures (e.g. grain
size of 5 nm), the ZT coefficient can be as high as 0.2 at room temperature,
which is the highest ZT value among all the Si based bulk nanostructures so
far. Such substantial improvement stems from two aspects: (1) the improvement
of the power factor is caused by the increase of Seebeck coefficient
(degeneracy of the band valley) and the enhancement of electrical conductivity
(the reduction of the effective band mass); (2) the significant reduction of
the lattice thermal conductivity is mainly caused by the extremely strong
phonon-grain boundary and phonon-twin boundary scattering.

###Thermoelectric properties of $β$-As, Sb and Bi monolayers|Dong-Chen Zhang,Ai-Xia Zhang,San-Dong Guo###

Thermoelectric properties of $β$-As, Sb and Bi monolayers. Monolayer semiconductors of group-VA elements (As, Sb, Bi) with graphenelike
buckled structure offer a potential to achieve nanoscale electronic,
optoelectronic and thermoelectric devices. Motivated by recently-fabricated Sb
monolayer, we systematically investigate the thermoelectric properties of
$\beta$-As, Sb and Bi monolayers by combining the first-principles calculations
and semiclassical Boltzmann transport theory. The generalized gradient
approximation (GGA) plus spin-orbit coupling (SOC) is adopted for the electron
part, and GGA is employed for the phonon part. It is found that SOC has
important influences on their electronic structures, especially for Bi
monolayer, which can induce observable SOC effects on electronic transport
coefficients. More specifically, SOC not only has detrimental influences on
electronic transport coefficients, but also produces enhanced effects. The
calculated lattice thermal conductivity decreases gradually from As to Bi
monolayer, and the corresponding room-temperature sheet thermal conductance is
161.10 $\mathrm{W K^{-1}}$, 46.62 $\mathrm{W K^{-1}}$ and 16.02 $\mathrm{W
K^{-1}}$, which can be converted into common lattice thermal conductivity by
dividing by the thickness of 2D material. The sheet thermal conductance of Bi
monolayer is lower than one of other 2D materials, such as semiconducting
transition-metal dichalcogenide monolayers and orthorhombic group IV-VI
monolayers. A series of scattering time is employed to estimate the
thermoelectric figure of merit $ZT$. It is found that the n-type doping has
more excellent thermoelectric properties than p-type doping for As and Bi
monolayer, while the comparative $ZT$ between n- and p-type doping is observed
in Bi monolayer. These results can stimulate further experimental works to open
the new field for thermoelectric devices based on monolayer of group-VA
elements.

###Potential 2D thermoelectric materials ATeI (A=Sb and Bi) monolayers from a first-principles study|San-Dong Guo,Ai-Xia Zhang###

Potential 2D thermoelectric materials ATeI (A=Sb and Bi) monolayers from a first-principles study. Lots of two-dimensional (2D) materials have been predicted theoretically, and
further confirmed in experiment, which have wide applications in nanoscale
electronic, optoelectronic and thermoelectric devices. Here, the thermoelectric
properties of ATeI (A=Sb and Bi) monolayers are systematically investigated,
based on semiclassical Boltzmann transport theory. It is found that spin-orbit
coupling (SOC) has important effects on electronic transport coefficients in
p-type doping, but neglectful influences on n-type ones. The room-temperature
sheet thermal conductance is 14.2 $\mathrm{W K^{-1}}$ for SbTeI and 12.6
$\mathrm{W K^{-1}}$ for BiTeI, which are lower than one of most well-known 2D
materials, such as transition-metal dichalcogenide, group IV-VI, group-VA and
group-IV monolayers. By analyzing group velocities and phonon lifetimes, the
very low sheet thermal conductance of ATeI (A=Sb and Bi) monolayers is mainly
due to small group velocities. It is found that the high-frequency optical
branches contribute significantly to the total thermal conductivity, being
obviously different from usual picture with little contribution from optical
branches. According to cumulative lattice thermal conductivity with respect to
phonon mean free path (MFP), it is difficulty to further reduce lattice thermal
conductivity by nanostructures. Finally, possible thermoelectric figure of
merit $ZT$ of ATeI (A=Sb and Bi) monolayers are calculated. It is found that
the p-type doping has more excellent thermoelectric properties than n-type
doping, and at room temperature, the peak $ZT$ can reach 1.11 for SbTeI and
0.87 for BiTeI, respectively. These results make us believe that ATeI (A=Sb and
Bi) monolayers may be potential 2D thermoelectric materials, and can stimulate
further experimental works to synthesize these monolayers.

###First principle design of new thermoelectrics from TiNiSn based pentanary alloys based on 18 valence electron rule|Mukesh K. Choudhary,P. Ravindran###

First principle design of new thermoelectrics from TiNiSn based pentanary alloys based on 18 valence electron rule. In this study, we have reported electronic structure, lattice dynamics, and
thermoelectric (TE) transport properties of a new family of pentanary
substituted TiNiSn systems using the 18 valence electron count (VEC) rule. From
our calculated band structures and density of states, we show that by
preserving the 18 VEC through aliovalent substitutions at the Ti site of TiNiSn
semiconducting behavior can be achieved, and hence one can tune the band
structure and band gap to maximize the thermoelectric figure of merit (ZT)
value. Two approaches have been used for calculating the lattice thermal
conductivity ($\kappa_{L}$), one by fully solving the linearized phonon
Boltzmann transport (LBTE) equation from first$-$principles anharmonic lattice
dynamics calculations implemented in Phono3py code and other using Slack's
equation with calculated Debye temperature and Gr\"{u}neisen parameter using
the calculated elastic constant values. The calculated $\kappa_{L}$ values
decrease from parent TiNiSn to pentanary substituted TiNiSn systems as expected
due to fluctuation in atomic mass. The calculated $\kappa_{L}$ for Hf
containing systems La$_{0.25}$Hf$_{0.5}$V$_{0.25}$NiSn and non Hf containing
system La$_{0.25}$Zr$_{0.5}$V$_{0.25}$NiSn calculated from Phono3py (Slack's
equation) are found to be 0.37 (1.04) and 0.16 (0.95) W/mK, at 550\,K,
respectively and the corresponding ZT value are found to be 0.54 (0.4) and 0.77
(0.53). Among the considered systems, the calculated phonon spectra and heat
capacity show that La$_{0.25}$Hf$_{0.5}$V$_{0.25}$NiSn has more
optical$-$acoustic band mixing which creates more phonon$-$phonon scattering
and hence lower the $\kappa_{L}$ value and maximizing the ZT. Based on the
calculated results we conclude that one can design high-efficiency
thermoelectric materials by considering the 18 VEC rule with aliovalent
substitution.

###Thermoelectric properties of junctions between metal and strongly correlated semiconductor|Massimo Rontani,L. J. Sham###

Thermoelectric properties of junctions between metal and strongly correlated semiconductor. We propose a junction of metal and rare-earth compound semiconductor as the
basis for a possible efficient low-temperature thermoelectric device. If an
overlayer of rare earth atoms differing from the bulk is placed at the
interface, very high values of the figure of merit ZT can be reached at low
temperature. This is due to sharp variation of the transmission coefficient of
carriers across the junction at a narrow energy range, which is intrinsically
linked to the localized character of the overlayer f-orbital.

###Theory of enhancement of thermoelectric properties of materials with nanoinclusions|Sergey V. Faleev,François Léonard###

Theory of enhancement of thermoelectric properties of materials with nanoinclusions. Based on the concept of band bending at metal/semiconductor interfaces as an
energy filter for electrons, we present a theory for the enhancement of the
thermoelectric properties of semiconductor materials with metallic
nanoinclusions. We show that the Seebeck coefficient can be significantly
increased due to a strongly energy-dependent electronic scattering time. By
including phonon scattering, we find that the enhancement of ZT due to electron
scattering is important for high doping, while at low doping it is primarily
due to a decrease in the phonon thermal conductivity.

###Effect of Chemical Doping on the Thermoelectric Properties of FeGa3|N. Haldolaarachchige,A. B. Karki,W. Adam Phelan,Y. M. Xiong,R. Jin,Julia Y. Chan,S. Stadler,D. P. Young###

Effect of Chemical Doping on the Thermoelectric Properties of FeGa3. Thermoelectric properties of the chemically-doped intermetallic narrow-band
semiconductor FeGa3 are reported. The parent compound shows semiconductor-like
behavior with a small band gap (Eg = 0.2 eV), a carrier density of ~ 10(18)
cm-3 and, a large n-type Seebeck coefficient (S ~ -400 \mu V/K) at room
temperature. Hall effect measurements indicate that chemical doping
significantly increases the carrier density, resulting in a metallic state,
while the Seebeck coefficient still remains fairly large (~ -150 \mu V/K). The
largest power factor (S2/{\rho} = 62 \mu W/m K2) and corresponding figure of
merit (ZT = 0.013) at 390 K were observed for Fe0.99Co0.01(Ga0.997Ge0.003)3.

###Phonon Drag Effect in Nanocomposite FeSb2|Mani Pokharel,Huaizhou Zhao,Kevin Lukas,Bogdan Mihaila,Zhifeng Ren,Cyril Opeil###

Phonon Drag Effect in Nanocomposite FeSb2. We study the temperature dependence of thermoelectric transport properties of
four FeSb2 nanocomposite samples with different grain sizes. The comparison of
the single crystals and nanocomposites of varying grain size indicates the
presence of substantial phonon drag effects in this system contributing to a
large Seebeck coefficient at low temperature. As the grain size decreases, the
increased phonon scattering at the grain boundaries leads to a suppression of
the phonon-drag effect, resulting in a much smaller peak value of the Seebeck
coefficient in the nanostructured bulk materials. As a consequence, the ZT
values are not improved significantly even though the thermal conductivity is
drastically reduced.

###Thermal Conductivity of Core-Shell Based Nanocomposites for Enhancing Thermoelectric ZT|S. J. Poon,A. S. Petersen,Di Wu###

Thermal Conductivity of Core-Shell Based Nanocomposites for Enhancing Thermoelectric ZT. The differential effective medium method (DEM) is presented from a physical
viewpoint and employed to calculate the lattice thermal conductivity of
nano-bulk composites comprising core-shell particles. Extended from the
average-T-matrix single-particle approximation, DEM incorporates interparticle
effect essential for the study of core-shell nanocomposites (CSN).
Interparticle boundary scattering in addition to intraparticle boundary
scattering in CSN is found to add to the reduction of thermal conductivity of
nanocomposites. Thus, CSN hold the promise of improving the thermoelectric
dimensionless figure of merit ZT above that of monolithic nano-bulk phases. Si
and SiGe based CSN serve as illustrative examples.

###Thermoelectric properties of doped small molecule organic semiconductor films|Torben Menke###

Thermoelectric properties of doped small molecule organic semiconductor films. Doped films of organic small molecules are investigated with respect to their
thermoelectric properties. A variety of hosts and dopants, for both n and
p-doping, are compared. C$_{60}$ n-doped by Cr$_2$(hpp)$_4$ or o-MeO-DMBI-I are
found to be the most promising material systems with a maximum of ZT$_\text{M}$
= 0.069 at T$_\text{M}$ = 40{\deg}C, assuming a doping-independent thermal
conductivity due to phonon-based heat transport. This value is 16% of the
current record reported for optimized devices employing the doped polymer
PEDOT:PSS.

###Graphdiyne: a two-dimensional thermoelectric material with high figure of merit|L. Sun,P. H. Jiang,H. J. Liu,D. D. Fan,J. H. Liang,J. Wei,L. Cheng,J. Zhang,J. Shi###

Graphdiyne: a two-dimensional thermoelectric material with high figure of merit. As a new carbon allotrope, the recently fabricated graphdiyne has attracted
much attention due to its interesting two-dimensional character. Here we
demonstrate by multiscale computations that, unlike graphene, graphdiyne has a
natural band gap, and simultaneously possess high electrical conductivity,
large Seebeck coefficient, and low thermal conductivity. At a carrier
concentration of 2.74*10^11/cm^2 for holes and 1.62*10^11/cm^2 for electrons,
the room temperature ZT value of graphdiyne can be optimized to 3.0 and 4.8,
respectively, which makes it an ideal system to realize the concept of
"phonon-glass and electron-crystal" in the thermoelectric community.

###High-temperature oxide thermoelectrics|Ichiro Terasaki###

High-temperature oxide thermoelectrics. We have evaluated the power factor of transition metal oxides at high
temperatures using the Heikes formula and the Ioffe-Regel conductivity. The
evaluated power factor is found to be nearly independent of carrier
concentration in a wide range of doping, and well explains the experimental
data for cobalt oxides. This suggests that the same power factor can be
obtained with a thermopower larger than $2k_B/e$, and also suggests a
reasonably high value of the dimensionless figure of merit $ZT$. We propose an
oxide thermoelectric power generator by using materials having a thermopower
larger than 300 $\mu$V/K.

###SPS-Sintered NaTaO3-Fe2O3 Composite exhibits Large Seebeck Coefficient and Electric Current|Wilfried Wunderlich,Takao Mori,Oksana Sologub###

SPS-Sintered NaTaO3-Fe2O3 Composite exhibits Large Seebeck Coefficient and Electric Current. NaTaO3-50wt% Fe2O3 composite ceramics showed a large Seebeck voltage of -300
mV at a temperature gradient of 650 K yielding a constant Seebeck coefficient
of more than -500 microV/K over a wide temperature range. We report for the
first time that SPS sintering at low temperature 870K could maintain the
short-circuit current of -80 microA, which makes this thermoelectric material a
possible candidate for high-temperature applications up to 1623 K. The reason
for the good performance is the interface between Fe2O3 and surrounding NaTaO3
perovskite. When spark-plasma sintering (SPS) is used, constitutional vacancies
disappeared and the electric conductivity increases remarkably yielding ZT of
0.016.

###Thermoelectric refrigerator based on asymmetric surfaces of a magnetic topological insulator|Takahiro Chiba,Takashi Komine###

Thermoelectric refrigerator based on asymmetric surfaces of a magnetic topological insulator. Thermoelectric (TE) refrigeration such as Peltier cooler enables a unique
opportunity in electric energy to directly convert thermal energy. Here, we
propose a TE module with both refrigeration and power generation modes by
utilizing asymmetric surfaces of a magnetic topological insulator (quantum
anomalous Hall insulator) with a periodic array of hollows filled with two
different dielectrics. Based on the Boltzmann transport theory, we show that
its efficiency, i.e., the dimensionless figure of merit ZT exceeds 1 in the
low-temperature regime below 300 K. The proposed device could be utilized as a
heat management device that requires precise temperature control in small-scale
cooling.

###Benefits of Carrier Pocket Anisotropy to Thermoelectric Performance: The case of $p$-type AgBiSe$_2$|David Parker,Andrew F. May,David J. Singh###

Benefits of Carrier Pocket Anisotropy to Thermoelectric Performance: The case of $p$-type AgBiSe$_2$. We study theoretically the effects of anisotropy on the thermoelectric
performance of $p$-type AgBiSe$_2$. We present an apparent realization of the
thermoelectric benefits of one-dimensional "plate-like" carrier pocket
anisotropy in the valence band of this material. Based on first principles
calculations we find a substantial anisotropy in the electronic structure,
likely favorable for thermoelectric performance, in the valence bands of the
hexagonal phase of the silver chalcogenide thermoelectric AgBiSe$_2$, while the
conduction bands are more isotropic, and in our experiments do not attain high
performance. AgBiSe$_2$ has already exhibited a $ZT$ value of 1.5 in a
high-temperature disordered fcc phase, but room-temperature performance has not
been demonstrated. We develop a theory for the ability of anisotropy to
decouple the density-of-states and conductivity effective masses, pointing out
the influence of this effect in the high performance thermoelectrics
Bi$_2$Te$_3$ and PbTe. From our first principles and Boltzmann transport
calculations we estimate the performance of $p$-type AgBiSe$_{2}$.

###Thermoelectric properties of topological insulator $\mathrm{BaSn_2}$|San-Dong Guo,Liang Qiu###

Thermoelectric properties of topological insulator $\mathrm{BaSn_2}$. Recently, $\mathrm{BaSn_2}$ is predicted to be a strong topological insulator
by the first-principle calculations. It is well known that topological
insulator has a close connection to thermoelectric material, such as
$\mathrm{Bi_2Te_3}$ family. In this work, we investigate thermoelectric
properties of $\mathrm{BaSn_2}$ by the first-principles combined with Boltzmann
transport theory. The electronic part is carried out by a modified Becke and
Johnson (mBJ) exchange potential, including spin-orbit coupling (SOC), while
the phonon part is performed using generalized gradient approximation (GGA). It
is found that the electronic transport coefficients between the in-plane and
cross-plane directions show the strong anisotropy, while lattice lattice
thermal conductivities show an almost isotropy. Calculated results show a very
low lattice thermal conductivity for $\mathrm{BaSn_2}$, and the corresponding
average lattice thermal conductivity at room temperature is 1.69 $\mathrm{W
m^{-1} K^{-1}}$, which is comparable or lower than those of lead chalcogenides
and bismuth-tellurium systems as classic thermoelectric materials. Due to the
complicated scattering mechanism, calculating scattering time $\tau$ is
challenging. By using a empirical $\tau$=$10^{-14}$ s, the n-type figure of
merit $ZT$ is greater than 0.40 in wide temperature range. Experimentally, it
is possible to attain better thermoelectric performance, or to enhance one by
strain or tuning size parameter. This work indicates that $\mathrm{BaSn_2}$ may
be a potential thermoelectric material, which can stimulate further theoretical
and experimental works.

###Search for Thermoelectrics with High Figure of Merit in half-Heusler compounds with multinary substitution|Mukesh K. Choudhary,P Ravindran###

Search for Thermoelectrics with High Figure of Merit in half-Heusler compounds with multinary substitution. In order to improve the thermoelectric performance of TiCoSb we have
substituted 50% of Ti equally with Zr and Hf at Ti site and Sb with Sn and Se
equally at Sb site. The electronic structure of Ti0.5Zr0.25Hf0.25CoSn0.5Se0.5
is investigated using the full potential linearized augmented plane wave method
and the thermoelectric transport properties are calculated on the basis of
semi-classical Boltzmann transport theory. Our band structure calculations show
that Ti0.5Zr0.25Hf0.25CoSn0.5Se0.5 has semiconducting behavior with indirect
band gap value of 0.98 eV which follow the empirical rule of 18
valence-electron content to bring semiconductivity in half Heusler compounds,
indicating that one can have semiconducting behavior in multinary phase of half
Heusler compounds if they full fill the 18 VEC rule and this open-up the
possibility of designing thermoelectrics with high figure of merit in half
Heusler compounds. We show that at high temperature of around 700K
Ti0.5Zr0.25Hf0.25CoSn0.5Se0.5 has high thermoelectric figure of merit of ZT =
1.05 which is higher than that of TiCoSb (~ 0.95) suggesting that by going from
ternary to multinary phase system one can enhance the thermoelectric figure of
merit at higher temperatures.

###Universal Behavior of the Thermoelectric Figure of Merit, zT, vs. Quality Factor|Evan Witkoske,Xufeng Wang,Jesse Maassen,Mark Lundstrom###

Universal Behavior of the Thermoelectric Figure of Merit, zT, vs. Quality Factor. To increase the performance of thermoelectric materials, the electronic
parameters in the figure of merit must be improved. In this paper, we use full,
numerical band structures and solve the Boltzmann equation in the relaxation
time approximation using energy-dependent scattering times informed by first
principles simulations. By varying the strength of the electron-phonon coupling
or the lattice thermal conductivity, we compute the thermoelectric figure of
merit, zT, vs. a generalized thermoelectric quality factor. More than a dozen
different complex electronic structures are examined. Surprisingly, we find
that at a given quality factor, none provides a better figure of merit than
that of a material with a simple, parabolic band and acoustic deformation
potential scattering. A qualitative argument for this unexpected finding is
presented. This apparent universal behavior suggests that even for complex
electronic band structures, the thermoelectric figure of merit depends solely
on the ratio of electrical to thermal conductivity; the Seebeck coefficient and
Lorenz number need not be considered. This observation should simplify the
search for promising new materials, but if exceptions to this behavior can be
identified, new paths for increasing thermoelectric material performance will
open up.

###Strain Induced Enhancement of Thermoelectric Properties of Monolayer WS2 through Valley Degeneracy|Jayanta Bera,Satyajit Sahu###

Strain Induced Enhancement of Thermoelectric Properties of Monolayer WS2 through Valley Degeneracy. Two-dimensional transition metal dichalcogenides show great potential as
promising thermoelectric materials due to their lower dimensionality, the
unique density of states and quantum confinement of carriers. The effect of
mechanical strain on the thermoelectric performances of monolayer WS 2 has been
investigated using density functional theory associated with semiclassical
Boltzmann transport theory. The variation of Seebeck coefficient and band gap
with applied strain has followed the same type of trend. For n-type material
the relaxation time scaled power factor(S 2 {\sigma}/{\tau}) increases by the
application of compressive strain whereas for p- type material it increases
with the application of tensile strain. A 77% increase in the power factor has
been observed for the n-type material by the application of uniaxial
compressive strain. A decrease in lattice thermal conductivity with the
increase in temperature causes an almost 40% increase in ZT product under
applied uniaxial compressive strain. From the study, it is observed that
uniaxial compressive strain is more effective among all types of strain to
enhance the thermoelectric performance of monolayer WS 2 . Such strain induced
enhancement of thermoelectric properties in monolayer WS 2 could open a new
window for the fabrication of high-quality thermoelectric devices.

###First-principles study of LaOPbBiS$_3$ and its analogous compounds as thermoelectric materials|Keiya Kurematsu,Masayuki Ochi,Hidetomo Usui,Kazuhiko Kuroki###

First-principles study of LaOPbBiS$_3$ and its analogous compounds as thermoelectric materials. LaOBiPbS$_3$ is a kind of pnictogen-dichalcogenide layered compounds, which
have recently been experimentally investigated as thermoelectric materials
owing to their low thermal conductivity and high controllability of constituent
elements. However, thermoelectric performance of LaOBiPbS$_3$ is at present not
very high and that of its analogous compounds remains to be unknown. In this
study, we theoretically investigate thermoelectric properties of 24 possible
variations of the constituent elements in LaOBiPbS$_3$ from the viewpoint of
the electronic structure. We find that some compounds can have much better
thermoelectric performance than LaOBiPbS$_3$; in particular, LaOSbPbSe$_3$ is
predicted to have a power factor five times as large as that of LaOBiPbS$_3$.
Here, the choice of the pnictogen atom (As, Sb, and Bi), of which the
low-energy conduction bands mainly consist, correlates with the calculated
power factor and the dimensionless figure of merit, $ZT$. Such correlation
comes from the fact that the low-dimensionality of the electronic structure,
which enhances the density of states near the band edge, strongly depends on
the pnictogen atom through, e.g., the strength of the spin-orbit coupling.
Moreover, hybridization of the wave functions in the pnictogen-dichalcogenide
layer and those in the rock-salt layer plays a key role in gap opening, and
thus is important for achieving high thermoelectric performance. In
LaOSbPbSe$_3$, such hybridization also pushes up the conduction band bottom,
which enhances the density of states near the band edge and thus the power
factor.

###Electronic Origin for the Enhanced Thermoelectric Efficiency of Cu2Se|Shucui Sun,Yiwei Li,Yujie Chen,Xiang Xu,Lu Kang,Jingsong Zhou,Wei Xia,Shuai Liu,Meixiao Wang,Juan Jiang,Aiji Liang,Ding Pei,Kunpeng Zhao,Pengfei Qiu,Xun Shi,Lidong Chen,Yanfeng Guo,Zhengguo Wang,Yan Zhang,Zhongkai Liu,Lexian Yang,Yulin Chen###

Electronic Origin for the Enhanced Thermoelectric Efficiency of Cu2Se. Thermoelectric materials (TMs) can uniquely convert waste heat into
electricity, which provides a potential solution for the global energy crisis
that is increasingly severe. Bulk Cu2Se, with ionic conductivity of Cu ions,
exhibits a significant enhancement of its thermoelectric figure of merit zT by
a factor of ~3 near its structural transition around 400 K. Here, we show a
systematic study of the electronic structure of Cu2Se and its temperature
evolution using high-resolution angle-resolved photoemission spectroscopy. Upon
heating across the structural transition, the electronic states near the corner
of the Brillouin zone gradually disappear, while the bands near the centre of
Brillouin zone shift abruptly towards high binding energies and develop an
energy gap. Interestingly, the observed band reconstruction well reproduces the
temperature evolution of the Seebeck coefficient of Cu2Se, providing an
electronic origin for the drastic enhancement of the thermoelectric performance
near 400 K. The current results not only bridge among structural phase
transition, electronic structures, and thermoelectric properties in a condensed
matter system, but also provide valuable insights into the search and design of
new generation of thermoelectric materials.

###Mechanical, optoelectronic and thermoelectric properties of half-Heusler p-type semiconductor BaAgP: A DFT investigation|F. Parvin,M. A. Hossain,M. I. Ahmed,K. Akter,A. K. M. A. Islam###

Mechanical, optoelectronic and thermoelectric properties of half-Heusler p-type semiconductor BaAgP: A DFT investigation. We have explored the mechanical, electronic, optical and thermoelectric
properties of p-type half-Heusler compound BaAgP for the first time using
density functional theory based calculations. The mechanical and dynamical
stability of this compound is confirmed by studying the Born stability criteria
and phonon dispersion curve, respectively. It is soft, ductile and elastically
anisotropic. The atomic bonding along a-axis is stronger than that along
c-axis. The calculated electronic structure reveals that the studied compound
is an indirect band gap semiconductor. The analysis of charge density
distribution map and Mulliken population reveals that the bonding in BaAgP is a
mixture of covalent and ionic. The optical features confirm that BaAgP is
optically anisotropic. The high absorption coefficient and low reflectivity in
the visible to ultraviolet region make this compound a possible candidate for
solar cell and optoelectronic device applications. The thermoelectric
properties have been evaluated by solving the Boltzmann semi-classical
transport equations. The calculated power factor at 1000K along a-axis is 35.2
micro-W/cmK2 (with tau=10-14 s) which is ~3.5 times larger than that of SnSe, a
promising layered thermoelectric materials. The thermoelectric figure of merit,
ZT of BaAgP is 0.44 which is small due to high thermal conductivity. So the
reduction of thermal conductivity is essential to enhance thermoelectric
performance of BaAgP in device applications.

###Enhancing thermoelectric properties of isotope graphene nanoribbons via machine learning guided manipulation of disordered antidots and interfaces|Xiang Huang,Shengluo Ma,Haidong Wang,Shangchao Lin,C. Y. Zhao,Hong Wang,Shenghong Ju###

Enhancing thermoelectric properties of isotope graphene nanoribbons via machine learning guided manipulation of disordered antidots and interfaces. Structural manipulation at the nanoscale breaks the intrinsic correlations
among different energy carrier transport properties, achieving high
thermoelectric performance. However, the coupled multifunctional (phonon and
electron) transport in the design of nanomaterials makes the optimization of
thermoelectric properties challenging. Machine learning brings convenience to
the design of nanostructures with large degree of freedom. Herein, we conducted
comprehensive thermoelectric optimization of isotopic armchair graphene
nanoribbons (AGNRs) with antidots and interfaces by combining Green's function
approach with machine learning algorithms. The optimal AGNR with ZT of 0.894 by
manipulating antidots was obtained at the interfaces of the aperiodic isotope
superlattices, which is 5.69 times larger than that of the pristine structure.
The proposed optimal structure via machine learning provides physical insights
that the carbon-13 atoms tend to form a continuous interface barrier
perpendicular to the carrier transport direction to suppress the propagation of
phonons through isotope AGNRs. The antidot effect is more effective than
isotope substitution in improving the thermoelectric properties of AGNRs. The
proposed approach coupling energy carrier transport property analysis with
machine learning algorithms offers highly efficient guidance on enhancing the
thermoelectric properties of low-dimensional nanomaterials, as well as to
explore and gain non-intuitive physical insights.

###Thermoelectric properties of Co, Ir, and Os-Doped FeSi Alloys: Evidence for Strong Electron-Phonon Coupling|Brian C. Sales,Olivier Delaire,Michael A. McGuire,Andrew F. May###

Thermoelectric properties of Co, Ir, and Os-Doped FeSi Alloys: Evidence for Strong Electron-Phonon Coupling. The effects of various transition metal dopants on the electrical and thermal
transport properties of Fe1-xMxSi alloys (M= Co, Ir, Os) are reported. The
maximum thermoelectric figure of merit ZTmax is improved from 0.007 at 60 K for
pure FeSi to ZT = 0.08 at 100 K for 4% Ir doping. A comparison of the thermal
conductivity data among Os, Ir and Co doped alloys indicates strong
electron-phonon coupling in this compound. Because of this interaction, the
common approximation of dividing the total thermal conductivity into
independent electronic and lattice components ({\kappa}Total =
{\kappa}electronic + {\kappa}lattice) fails for these alloys. The effects of
grain size on thermoelectric properties of Fe0.96Ir0.04Si alloys are also
reported. The thermal conductivity can be lowered by about 50% with little or
no effect on the electrical resistivity or Seebeck coefficient. This results in
ZTmax = 0.125 at 100 K, still about a factor of five too low for solid-state
refrigeration applications.

###A Structural Phase Transition in Ca3Co4O9 Associated with Enhanced High Temperature Thermoelectric Properties|Tao Wu,Trevor A. Tyson,Haiyan Chen,Jianming Bai,Hsin Wang,Cherno Jaye###

A Structural Phase Transition in Ca3Co4O9 Associated with Enhanced High Temperature Thermoelectric Properties. Temperature dependent electrical resistivity, crystal structure and heat
capacity measurements reveal a resistivity drop and metal to semiconductor
transition corresponding to first order structural phase transition near 400 K
in Ca3Co4O9. The lattice parameter c varies smoothly with increasing
temperature, while anomalies in the a, b1 and b2 lattice parameters occur at ~
400 K. Both Ca2CoO3 and CoO2 layers become distorted above ~ 400 K associated
with the metal to semiconductor transport behavior change. Resistivity and heat
capacity measurements as a function of temperature under magnetic field
indicates low spin contribution to this transition. Reduced resistivity
associated with this first order phase transition from metallic to
semiconducting behavior enhances the thermoelectric properties at high
temperatures and points to the metal to semiconductor transition as a mechanism
for improved ZT in high temperature thermoelectric oxides.

###Thermoelectric effect enhanced by the resonant states in graphene|M. Inglot,A. Dyrdał,V. K. Dugaev,J. Barnaś###

Thermoelectric effect enhanced by the resonant states in graphene. Thermoelectric effects in graphene are considered theoretically with
particular attention paid to the role of impurities. Using the T -matrix method
we calculate the impurity resonant states and the momentum relaxation time due
to scattering on impurities. The Boltzmann kinetic equation is used to
determine the thermoelectric coefficients. It is shown that the resonant
impurity states near the Fermi level give rise to a resonant enhancement of the
Seebeck coefficient and of the figure of merit $ZT$ . The Wiedemann-Franz ratio
deviates from that known for ordinary metals, where this ratio is constant and
equal to the Lorentz number. This deviation appears for small chemical
potentials and in the vicinity of the resonant states. In the limit of a
constant relaxation time, this ratio has been calculated analytically for
$\mu=0$.

###Thermoelectric Properties of Nanoscale three dimensional Si Phononic Crystal|Lina Yang,Nuo Yang,Baowen Li###

Thermoelectric Properties of Nanoscale three dimensional Si Phononic Crystal. The thermoelectric properties of n type nanoscale three dimensional (3D) Si
phononic crystals (PnCs) with spherical pores are studied. Density functional
theory and Boltzmann transport equation under the relaxation time approximation
are applied to study the electronic transport coefficients, electrical
conductivity, Seebeck coefficient and electronic thermal conductivity. We found
that the electronic transport coefficients in 3D Si PnC at room temperature
(300 K) change very little compared with that of Si, for example, electrical
conductivity and electronic thermal conductivity is decreased by 0.26 to 0.41
and 0.39 to 0.55 depending on carrier concentration, respectively, and the
Seebeck coefficient is similar to that of bulk Si. However, the lattice thermal
conductivity of 3D Si PnCs with spherical pores is decreased by a factor of 500
calculated by molecular dynamics methods, leading to the ZT of 0.76, which is
about 30 times of that of porous Si. This work indicates that 3D Si PnC is a
promising candidate for high efficiency thermoelectric materials.

###Enhanced thermopower and low thermal conductivity in p-type polycrystalline ZrTe5|M. K. Hooda,C. S. Yadav###

Enhanced thermopower and low thermal conductivity in p-type polycrystalline ZrTe5. Thermoelectric properties of polycrystalline p-type ZrTe5 are reported in
temperature (T) range 2 - 340 K. Thermoelectric power (S) is positive and
reaches up to 458 uV/K at 340 K on increasing T. The value of Fermi energy 16
meV, suggests low carrier density of ~ 9.5 X 10^18 cm-3. A sharp anomaly in S
data is observed at 38 K, which seems intrinsic to p-type ZrTe5. The thermal
conductivity value is low (2 W/m-K at T = 300 K) with major contribution from
lattice part. Electrical resistivity data shows metal to semiconductor
transition at T ~ 150 K and non-Arrhenius behavior in the semiconducting
region. The figure of merit zT (0.026 at T = 300 K) is ~ 63% higher than HfTe5
(0.016), and better than the conventional SnTe, p-type PbTe and bipolar
pristine ZrTe5 compounds.

###Stoichiometric and off-stoichiometric full Heusler $\mathbf {Fe_2V_{1-x}W_xAl} $ thermoelectric systems|B. Hinterleitner,P. Fuchs,J. Rehak,S. Steiner,M. Kishimoto,R. Moser,R. Podloucky,E. Bauer###

Stoichiometric and off-stoichiometric full Heusler $\mathbf {Fe_2V_{1-x}W_xAl} $ thermoelectric systems. A series of full-Heusler alloys, $\rm Fe_2V_{1-x}W_xAl$, $0 \leq x \leq 0.2$,
was prepared, characterized and relevant physical properties to account for the
thermoelectric performance were studied in a wide temperature range.
Additionally, off-stoichiometric samples with similar compositions have been
included, and a 10~\% improvement of the thermoelectric figure of merit was
obtained. The V/W substitution causes i) a change of the main carrier type,
from holes to electrons as evidenced from Seebeck and Hall measurements and ii)
a substantial reduction of the lattice thermal conductivity due to a creation
of lattice disorder by means of a distinct different mass and metallic radius
upon the V/W substitution. Moreover $ZT$ values above 0.2 have been obtained. A
microscopic understanding of the experimental data observed is revealed from
ab-initio calculations of the electronic and phononic structure.

###Tuning the carrier concentration to improve the thermoelectric performance of CuInTe2 compound|J. Wei,H. J. Liu,L. Cheng,J. Zhang,J. H. Liang,P. H. Jiang,D. D. Fan,J. Shi###

Tuning the carrier concentration to improve the thermoelectric performance of CuInTe2 compound. The electronic and transport properties of CuInTe2 chalcopyrite are
investigated using density functional calculations combined with Boltzmann
theory. The band gap predicted from hybrid functional is 0.92 eV, which agrees
well with experimental data and leads to relatively larger Seebeck coefficient
compared with those of narrow-gap thermoelectric materials. By fine tuning the
carrier concentration, the electrical conductivity and power factor of the
system can be significantly optimized. Together with the inherent low thermal
conductivity, the ZT values of CuInTe2 compound can be enhanced to as high as
1.72 at 850 K, which is obviously larger than those measured experimentally and
suggests there is still room to improve the thermoelectric performance of this
chalcopyrite compound.

###First principles search for $n$-type oxide, nitride, and sulfide thermoelectrics|Kevin F. Garrity###

First principles search for $n$-type oxide, nitride, and sulfide thermoelectrics. Oxides have many potentially desirable characteristics for thermoelectric
applications, including low cost and stability at high temperatures, but thus
far there are few known high $zT$ $n$-type oxide thermoelectrics. In this work,
we use high-throughput first principles calculations to screen transition metal
oxides, nitrides, and sulfides for candidate materials with high power factors
and low thermal conductivity. We find a variety of promising materials, and we
investigate these materials in detail in order to understand the mechanisms
that cause them to have high power factors. These materials all combine a high
density of states near the Fermi level with dispersive bands, reducing the
trade-off between the Seebeck coefficient and the electrical conductivity, but
they do so for several different reasons. In addition, our calculations
indicate that many of our candidate materials have low thermal conductivity.

###Thermoelectric properties of AgGaTe$_2$ and related chalcopyrite structure materials|David Parker,David J. Singh###

Thermoelectric properties of AgGaTe$_2$ and related chalcopyrite structure materials. We present an analysis of the potential thermoelectric performance of p-type
AgGaTe$_{2}$, which has already shown a $ZT$ of 0.8 with partial optimization,
and observe that the same band structure features, such as a mixture of light
and heavy bands and isotropic transport, that lead to this good performance are
present in certain other ternary chalcopyrite structure semiconductors. We find
that optimal performance of AgGaTe$_2$ will be found for hole concentrations
between 4 $\times 10^{19}$ and 2 $\times 10^{20}$cm$^{-3}$ at 900 K, and 2
$\times 10^{19}$ and 10$^{20}$ cm$^{-3}$ at 700 K, and that certain other
chalcopyrite semiconductors might show good thermoelectric performance at
similar doping ranges and temperatures if not for higher lattice thermal
conductivity.

###Improvement in thermoelectric properties by tailoring at In and Te site in In2Te5|Anup V. Sanchela,Ajay D. Thakur,C. V. Tomy###

Improvement in thermoelectric properties by tailoring at In and Te site in In2Te5. We study role of site substitutions at In and Te site in In2Te5 on the
thermoelectric behavior. Single crystals with compositions In2(Te1-xSex)5 (x =
0, 0.05, 0.10) and Fe0.05In1.95(Te0.90Se0.10)5 were prepared using modified
Bridgman-Stockbarger technique. Electrical and thermal transport properties of
these single crystals were measured in the temperature range 6 - 395 K. A
substantial decrease in thermal conductivity is observed in Fe substituted
samples attributed to the enhanced phonon point-defect scattering. Marked
enhancement in Seebeck coefficient S along with a concomitant suppression of
electrical resistivity \r{ho} is observed in Se substituted single crystals. An
overall enhancement of thermoelectric figure of merit (zT) by a factor of 310
is observed in single crystals of Fe0.05In1.95(Te0.90Se0.10)5 compared to the
parent In2Te5 single crystals.

###High Thermoelectric Performance of Au@Sb2Te3 Heterostructure Derived from the Potential Barriers|Wenwen Zheng,Peng Bi,Fengming Liu,Yong Liu,Jing Shi,Rui Xiong,Ziyu Wang###

High Thermoelectric Performance of Au@Sb2Te3 Heterostructure Derived from the Potential Barriers. The correlated couple of electrical and thermal property is the challenge to
realize a substantial leap in thermoelectric materials.Synthesis of
semiconductor and metal composites is a significant and versatile design
strategy to optimize the thermoelectric performance driven by tailored
interface between nanoinclusions and matrix.In this study, we present the
simultaneous increase of electrical conductivity and Seebeck coefficient, and
reduction of thermal conductivity in Sb2Te3-Au system.The enhanced electrical
conductivity lies in the incorporated Au nanostructures contributing to
injecting carriers to Sb2Te3 matrix.The appropriate barriers originated from
the Au-Sb2Te3 interface, which filter low energy carriers, results in
enhancement of Seebeck coefficient.The increased boundaries and nanodomains
block the transport of phonons, subsequently reducing the thermal
conductivity.As a consequence, combination of these effects promote double of
ZT value in 1% Au@ Sb2Te3 composites with respect to the pristine Sb2Te3.

###Lifshitz transition and thermoelectric properties of bilayer graphene|Dominik Suszalski,Grzegorz Rut,Adam Rycerz###

Lifshitz transition and thermoelectric properties of bilayer graphene. This is a numerical study of thermoelectric properties of ballistic bilayer
graphene in the presence of trigonal warping term in the effective Hamiltonian.
We find, in the mesoscopic samples of the length $L>10\,\mu{}$m at sub-Kelvin
temperatures, that both the Seebeck coefficient and the Lorentz number show
anomalies (the additional maximum and minimum, respectively) when the
electrochemical potential is close to the Lifshitz energy, which can be
attributed to the presence of the van Hove singularity in a bulk density of
states. At higher temperatures the anomalies vanish, but measurable quantities
characterizing remaining maximum of the Seebeck coefficient still unveil the
presence of massless Dirac fermions and make it possible to determine the
trigonal warping strength. Behavior of the thermoelectric figure of merit
($ZT$) is also discussed.

###A comparative study of the thermoelectric performance of graphene-like BX (X= P, As, Sb) monolayers|Z. Z. Zhou,H. J. Liu,D. D. Fan,G. H. Cao###

A comparative study of the thermoelectric performance of graphene-like BX (X= P, As, Sb) monolayers. The electronic and phonon transport properties of graphene-like boron
phosphide (BP), boron arsenide (BAs), and boron antimonide (BSb) monolayers are
investigated using first-principles calculations and Boltzmann theory. By
considering both the phonon-phonon and electron-phonon scatterings, we
demonstrate that the strong bond anharmonicity in the BAs and BSb monolayers
can dramatically suppress the phonon relaxation time but hardly affects that of
electrons. As a consequence, both systems exhibit comparable power factors with
that of the BP monolayer but much lower lattice thermal conductivities.
Accordingly, a maximum ZT values above 3.0 can be achieved in both BAs and BSb
monolayers at optimized carrier concentrations. Interestingly, very similar p-
and n-type thermoelectric performance is observed in the BSb monolayer along
the armchair direction, which is of vital importance in the fabrication of
thermoelectric modules with comparable efficiencies.

###Investigation of Thermoelectric properties of Magnetic Insulator FeRuTiSi Using First Principle Calculation|Saurabh Singh,Shubham Singh,Nitinkumar Bijewar,Ashish Kumar###

Investigation of Thermoelectric properties of Magnetic Insulator FeRuTiSi Using First Principle Calculation. In this work, we have investigated the electronic structure and
thermoelectric properties of quaternary heusler alloy, FeRuTiSi, using first
principle DFT tools implemented in WIEN2k and BoltzTraP code. Electronic
structure calculations using TB-mBJ potential shows appearance of flat band at
the conduction band edge, thus electron in conduction band have the large
effective mass (me*), and therefore mainly contribute for negatively large
value of Seebeck coefficient (S). This alloy has indirect band gap of 0.59 eV,
and shows the n-type transport behavior. Under the constant relaxation time
approximation (tau = 10 -14 s), temperature dependent Seebeck coefficient,
electrical conductivity (sigma), and electronic thermal conductivity (ke) were
also estimated. The maximum figure-of-merit (ZT), for the FeRuTiSi compound is
found to be ~0.86 at 840 K, with n-type doping, which suggests that this
quaternary alloy can be a good candidate among the n-type material for
thermoelectric applications in high-temperature reg

###Thermoelectric properties of graphyne from first-principles calculations|P. H. Jiang,H. J. Liu,L. Cheng,D. D. Fan,J. Zhang,J. Wei,J. H. Liang###

Thermoelectric properties of graphyne from first-principles calculations. The two-dimensional graphene-like carbon allotrope, graphyne, has been
recently fabricated and exhibits many interesting electronic properties. In
this work, we investigate the thermoelectric properties of {\gamma}-graphyne by
performing first-principles calculations combined with Boltzmann transport
theory for both electron and phonon. The carrier relaxation time is accurately
evaluated from the ultra-dense electron-phonon coupling matrix elements
calculated by adopting the density functional perturbation theory and Wannier
interpolation, rather than the generally used deformation potential theory
which only considers the electron-acoustic phonon scattering. It is found that
the thermoelectric performance of {\gamma}-graphyne exhibits a strong
dependence on the temperature and carrier type. At an intermediate temperature
of 600 K, a maximum ZT value of 1.5 and 1.0 can be achieved for the p- and
n-type systems, respectively.

###On the Calculation of Lorenz Numbers for Complex Thermoelectric Materials|Xufeng Wang,Vahid Askarpour,Jesse Maassen,Mark Lundstrom###

On the Calculation of Lorenz Numbers for Complex Thermoelectric Materials. A first-principles informed approach to the calculation of Lorenz numbers for
complex thermoelectric materials is presented and discussed. Example
calculations illustrate the importance of using accurate band structures and
energy-dependent scattering times. Results obtained by assuming that the
scattering rate follows the density-of-states show that in the non-degenerate
limit, Lorenz numbers below the commonly assumed lower limit of 2(kB/q)^2 can
occur. The physical cause of low Lorenz numbers is explained by the shape of
the transport distribution. The numerical and physical issues that need to be
addressed in order to produce accurate calculations of the Lorenz number are
identified. The results of this study provide a general method that should
contribute to the interpretation of measurements of total thermal conductivity
and to the search for materials with low Lorenz numbers, which may provide
improved thermoelectric figures of merit, zT.

###Enhanced figure of merit in nanostructured (Bi, Sb) 2 Te 3 with optimized composition, prepared by a straightforward arc-melting procedure|F Serrano-Sánchez,M Gharsallah,NM Nemes,N Biskup,M Varela,JL Martínez,MT Fernández-Díaz,JA Alonso###

Enhanced figure of merit in nanostructured (Bi, Sb) 2 Te 3 with optimized composition, prepared by a straightforward arc-melting procedure. Sb-doped Bi2Te3 is known since the 1950s as the best thermoelectric material
for near-room temperature operation. Improvements in material performance are
expected from nanostructuring procedures. We present a straightforward and fast
method to synthesize already nanostructured pellets that show an enhanced ZT
due to a remarkably low thermal conductivity and unusually high Seebeck
coefficient for a nominal composition optimized for arc-melting:
Bi0.35Sb1.65Te3. We provide a detailed structural analysis of the Bi2-xSbxTe3
series based on neutron powder diffraction as a function of composition and
temperature that reveals the important role played by atomic vibrations.
Arc-melting produces layered platelets with less than 50 nm-thick sheets. The
low thermal conductivity is attributed to the phonon scattering at the grain
boundaries of the nanosheets. This is a fast and cost-effective production
method of highly efficient thermoelectric materials.

###Solid-state Janus nanoprecipitation enables amorphous-like heat conduction in crystalline Mg3Sb2-based thermoelectric materials|Rui Shu,Zhijia Han,Anna Elsukova,Yongbin Zhu,Peng Qin,Feng Jiang,Jun Lu,Per O. Å. Persson,Justinas Palisaitis,Arnaud le Febvrier,Wenqing Zhang,Oana Cojocaru-Mirédin,Yuan Yu,Per Eklund,Weishu Liu###

Solid-state Janus nanoprecipitation enables amorphous-like heat conduction in crystalline Mg3Sb2-based thermoelectric materials. Solid-state precipitation can be used to tailor materials properties, ranging
from ferromagnets and catalysts to mechanical strengthening and energy storage.
Thermoelectric properties can be modified by precipitation to enhance phonon
scattering while retaining charge-carrier transmission. Here, we uncover
unconventional dual Janus-type nanoprecipitates in Mg3Sb1.5Bi0.5 formed by
side-by-side Bi- and Ge-rich appendages, in contrast to separate
nanoprecipitate formation. These Janus nanoprecipitates result from local
co-melting of Bi and Ge during sintering, enabling an amorphous-like lattice
thermal conductivity. A precipitate size effect on phonon scattering is
observed due to the balance between alloy-disorder and nanoprecipitate
scattering. The thermoelectric figure-of-merit ZT reaches 0.6 near room
temperature and 1.6 at 773 K. The Janus nanoprecipitation can be introduced
into other materials and may act as a general property-tailoring mechanism.

###Superparamagnetic and metal-like Ru2TiGe: a propitious thermoelectric material|Sanchayita Mondal,Krishanu Ghosh,R. Ranganathan,Eric Alleno,Chandan Mazumdar###

Superparamagnetic and metal-like Ru2TiGe: a propitious thermoelectric material. We report a study of structural, magnetic, heat capacity and thermoelectric
properties of a Rubased Heusler alloy, Ru2TiGe. The magnetic measurements
reveal that at higher temperatures, diamagnetic and Pauli paramagnetic
contributions dominate the magnetic behaviour whereas, at lower temperatures
(T<= 20 K), superparamagnetic interaction among clusters is observed. Effect of
such magnetic defects is also evident in the electrical resistivity behaviour
at lower temperatures. Though the temperature dependence of resistivity
exhibits a metal-like nature, the large value of Seebeck coefficient leads to
an appreciable power factor of the order of 1 mW/mK2 at 300 K. Large power
factor as well as low thermal conductivity results in a value of ZT = 0.025 at
390 K for Ru2TiGe that is orders of magnitude higher than that of the other
pure Heusler alloys and point towards its high potential for practical
thermoelectric applications.

###Mechanical, Optical and Thermoelectric Properties of Janus BiTeCl Monolayer|Poonam Chauhan,Jaspreet Singh,Ashok Kumar###

Mechanical, Optical and Thermoelectric Properties of Janus BiTeCl Monolayer. We report mechanical, optical and thermoelectric properties of recently
fabricated Janus BiTeCl monolayer using density functional and semi-classical
Boltzmann transport theory. Janus BiTeCl monolayer exhibits a direct bandgap,
high carrier mobility (~10$^3$ cm$^2$V$^{-1}$s$^{-1}$) and high optical
absorption in the UV-visible region. The mechanical behavior of the Janus
BiTeCl monolayer is nearly isotropic having an ideal tensile strength ~ 15 GPa.
The higher value of the Gruneisen parameter ($\gamma$), a low value of phonon
group velocity (vg), and very little phonon scattering time ($\tau_p$) lead to
low lattice thermal conductivity (1.46 W/mK) of Janus BiTeCl monolayer. The
combined effect of thermal conductivity and electronic transport coefficients
of Janus BiTeCl monolayer results in the figure of merit (ZT) in the range of
0.43-0.75 at 300-500 K. Our results suggest Janus BiTeCl monolayer be a
potential candidate for optoelectronic and moderate temperature thermoelectric
applications.

###Oxide perovskite BaSnO3: A promising high-temperature thermoelectric material for transparent conducting oxides|Xiefei Song,Guangzhao Wang,Li Zhou,Haiyan Yang,Xiaopan Li,Haitao Yang,Yuncheng Shen,Guangyang Xu,Yuhui Luo,Ning Wang###

Oxide perovskite BaSnO3: A promising high-temperature thermoelectric material for transparent conducting oxides. The new technology of energy conversion must be developed to ensure energy
sustainability. Thermoelectric (TE) materials provide an effective means to
solve the energy crisis. As a potential TE candidate, the TE properties of
perovskite have received extensively attention. We here investigate the TE
transport properties of the transparent conducting oxide (TCO) BaSnO3 by
first-principles calculations. We find that the BaSnO3 perovskite exhibits
outstanding dynamic and thermal stabilities, which provide excellent electronic
and thermal transport properties simultaneously. These properties contribute to
the remarkable Seebeck coefficient and power factor, which gives rise to the ZT
of n-1.03 and p-3.64 at 900 K. Additionally, doping and nanostructure open
prospects for effectively improving the TE properties of BaSnO3. Our work
provides a basis for further optimizing the TE transport properties of cubic
BaSnO3 and may have worthwhile practical significance for applying cubic
perovskite to the high-temperature thermoelectric field.

###Significant low lattice thermal conductivity and potential high thermoelectric figure of merit in Na$_2$MgSn|Cong Wang,Y. B. Chen,Shu-Hua Yao,Jian Zhou###

Significant low lattice thermal conductivity and potential high thermoelectric figure of merit in Na$_2$MgSn. Thermoelectric materials enables the harvest of waste heat and directly
conversion into electricity. In search of high efficient thermoelectric
materials, low thermal conductivity of a material is essential and critical.
Here, we have theoretically investigated the lattice thermal conductivity and
thermoelectric properties of layered intermetallic Na$_2$MgSn and Na$_2$MgPb
based on the density functional theory and linearized Boltzmann equation with
the single-mode relaxation-time approximation. It is found that both materials
exhibit very low and anisotropic intrinsic lattice thermal conductivity.
Despite of the very low mass density and simple crystal structure of
Na$_2$MgSn, its lattice thermal conductivities along $a$ and $c$ axes are only
1.75 and 0.80 W/m$\cdot$K respectively at room temperatures. When Sn is
replaced by the heavier element Pb, its lattice thermal conductivities decrease
remarkably to 0.51 and 0.31 W/m$\cdot$K respectively along $a$ and $c$ axes at
room temperatures. We show that the low lattice thermal conductivities of both
materials are mainly due to their very short phonon lifetimes, which are
roughly between 0.4 to 4.5 ps. Combined with previous experimental
measurements, the metallic Na$_2$MgPb can not be a good thermoelectric
material. However, we predict that the semiconducting Na$_2$MgSn is a potential
room-temperature thermoelectric material with a considerable $ZT$ of 0.34 at
300 K. Our calculations not only imply that the intermetallic Na$_2$MgSn is a
potential thermoelectric material, but also can motivate more theoretical and
experimental works on the thermoelectric researches in simple layered
intermetallic compounds.

###Thermoelectric transport properties of CaMg2Bi2, EuMg2Bi2, and YbMg2Bi2|A. F. May,M. A. McGuire,J. Ma,O. Delaire,A. Huq,D. J. Singh,W. Cai,H. Wang###

Thermoelectric transport properties of CaMg2Bi2, EuMg2Bi2, and YbMg2Bi2. The thermoelectric transport properties of CaMg2Bi2, EuMg2Bi2, and YbMg2Bi2
were characterized between 2 and 650K. As synthesized, the polycrystalline
samples are found to have lower p-type carrier concentrations than
single-crystalline samples of the same empirical formula. These low carrier
concentration samples possess the highest mobilities yet reported for materials
with the CaAl2Si2 structure type, with a mobility of ~740cm$^2$/V/s observed in
EuMg2Bi2 at 50K. Despite decreases in the Seebeck coefficient (\alpha) and
electrical resistivity (\rho) with increasing temperature, the power factor
(\alpha^2/\rho) increases for all temperatures examined. This behavior suggests
a strong asymmetry in the conduction of electrons and holes. The highest figure
of merit (zT) is observed in YbMg2Bi2, with zT approaching 0.4 at 600K for two
samples with carrier densities of approximately 2x10^{18}cm^{-3} and
8x10^{18}cm^{-3} at room temperature. Refinements of neutron powder diffraction
data yield similar behavior for the structures of CaMg2Bi2 and YbMg2Bi2, with
smooth lattice expansion and relative expansion in $c$ being ~35% larger than
relative expansion in $a$ at 973K. First principles calculations reveal an
increasing band gap as Bi is replaced by Sb then As, and subsequent Boltzmann
transport calculations predict an increase in \alpha for a given $n$ associated
with an increased effective mass as the gap opens. The magnitude and
temperature dependence of \alpha suggests higher zT is likely to be achieved at
larger carrier concentrations, roughly an order of magnitude higher than those
in the current polycrystalline samples, which is also expected from the
detailed calculations.

###Large thermoelectric response in a diluted ferroelectric system: Ba0.7Eu0.3Ti1-xNbxO3|KmRubi,R. Mahendiran###

Large thermoelectric response in a diluted ferroelectric system: Ba0.7Eu0.3Ti1-xNbxO3. We investigated the electrical conductivity, thermal conductivity and
thermopower as a function of Nb content (x) in Ba0.7Eu0.3Ti1-xNbxO3 (x = 0.001-
0.10) in the temperature range T = 400-2 K. The substitution of Nb destabilizes
the ferroelectric insulating ground state of Ba0.7Eu0.3TiO3 and transforms into
a paramagnetic metal for x = 0.1. Thermopower is negative in the entire
composition range (S = -613 microVolt/K at 400 K for x = 0.001) and its
magnitude decreases with increasing Nb content which suggests doping of
electrons into empty Ti-3d(t2g) conduction band. In this series, the
dimensionless figure of merit (ZT) increases with temperature for all the
compositions and the x = 0.03 composition exhibits the maximum ZT (= 0.12 at
400 K). The enhanced value of ZT is primarily due to the low thermal
conductivity of samples in this series (~ 0.7 to 1 W/(m.K) at 400 K) compared
to other potential high temperature n-type thermoelectric oxides such as
carrier doped SrTiO3 and CaMnO3. The low thermal conductivity in our compounds
most likely arises from heavy Eu2+ ion and lattice disorder introduced by Nb5+
which scatter phonons effectively.

###Ultrathin GaN Nanowires: Electronic, Thermal, and Thermoelectric Properties|A. H. Davoody,E. B. Ramayya,L. N. Maurer,I. Knezevic###

Ultrathin GaN Nanowires: Electronic, Thermal, and Thermoelectric Properties. We present a comprehensive computational study of the electronic, thermal,
and thermoelectric (TE) properties of gallium nitride nanowires (NWs) over a
wide range of thicknesses (3--9 nm), doping densities ($10^{18}$--$10^{20}$
cm$^{-3}$), and temperatures (300--1000 K). We calculate the low-field electron
mobility based on ensemble Monte Carlo transport simulation coupled with a
self-consistent solution of the Poisson and Schr\"odinger equations. We use the
relaxation-time approximation and a Poisson-Schro\"dinger solver to calculate
the electron Seebeck coefficient and thermal conductivity. Lattice thermal
conductivity is calculated using a phonon ensemble Monte Carlo simulation, with
a real-space rough surface described by a Gaussian autocorrelation function.
Throughout the temperature range, the Seebeck coefficient increases while the
lattice thermal conductivity decreases with decreasing wire cross section, both
boding well for TE applications of thin GaN NWs. However, at room temperature
these benefits are eventually overcome by the detrimental effect of surface
roughness scattering on the electron mobility in very thin NWs. The highest
room-temperature $ZT$ of 0.2 is achieved for 4-nm-thick NWs, while further
downscaling degrades it. In contrast, at 1000 K, the electron mobility varies
weakly with the NW thickness owing to the dominance of polar optical phonon
scattering and multiple subbands contributing to transport, so $ZT$ increases
with increasing confinement, reaching 0.8 for optimally doped 3-nm-thick NWs.
The $ZT$ of GaN NWs increases with increasing temperature beyond 1000 K, which
further emphasizes their suitability for high-temperature TE applications.

###High Thermoelectric Figure of Merit by Resonant Dopant in Half-Heusler Alloys|Long Chen,Yamei Liu,Jian He,Terry M. Tritt,S. Joseph Poon###

High Thermoelectric Figure of Merit by Resonant Dopant in Half-Heusler Alloys. Half-Heusler alloys have been one of the benchmark high temperature
thermoelectric materials owing to their thermal stability and promising figure
of merit ZT. Simonson et al. early showed that small amounts of vanadium doped
in Hf0.75Zr0.25NiSn enhanced the Seebeck coefficient and correlated the change
with the increased density of states near the Fermi level. We herein report a
systematic study on the role of vanadium (V), niobium (Nb), and tantalum (Ta)
as prospective resonant dopants in enhancing the ZT of n-type half-Heusler
alloys based on Hf0.6Zr0.4NiSn0.995Sb0.005. The V doping was found to increase
the Seebeck coefficient in the temperature range 300-1000 K, consistent with a
resonant doping scheme. In contrast, Nb and Ta act as normal n-type dopants, as
evident by the systematic decrease in electrical resistivity and Seebeck
coefficient. The combination of enhanced Seebeck coefficient due to the
presence of V resonant states and the reduced thermal conductivity has led to a
state-of-the-art ZT of 1.3 near 850 K in n-type
(Hf0.6Zr0.4)0.99V0.01NiSn0.995Sb0.005 alloys.

###Recent Advances in Thermoelectric Performance of Half-Heusler Compounds|S. Joseph Poon###

Recent Advances in Thermoelectric Performance of Half-Heusler Compounds. Half-Heusler phases (space group F43m, C1b) have recently captured much
attention as promising thermoelectric materials for heat-to-electric power
conversion in the mid-to-high temperature range. The most studied ones are the
RNiSn-type half-Heusler compounds, where R represents refractory metals Hf, Zr,
and Ti. These compounds have shown a high-power factor and high-power density,
as well as good material stability and scalability. Due to their high thermal
conductivity, however, the dimensionless figure of merit (zT) of these
materials has stagnated near 1 for a long time. Since 2013, the verifiable ZT
of half-Heusler compounds has risen from 1 to near 1.5 for both n- and p-type
compounds in the temperature range of 500 to 900 degrees C. In this brief
review, we summarize recent advances as well as approaches in achieving the
high ZT reported. In particular, we discuss the less-exploited strain-relief
effect and dopant resonant state effect studied by the author and his
collaborators in more detail. Finally, we point out directions for further
development.
  Keywords: half-Heusler compounds; figure of merit; power density; lattice
disorder; dopant resonant states

###Potential thermoelectric materials $\mathrm{CsMI_3}$ (M=Sn and Pb) in perovskite structures from the first-principles calculations|San-Dong Guo,Jian-Li Wang###

Potential thermoelectric materials $\mathrm{CsMI_3}$ (M=Sn and Pb) in perovskite structures from the first-principles calculations. The thermoelectric properties of halide perovskites $\mathrm{CsMI_3}$ (M=Sn
and Pb) are investigated from a combination of first-principles calculations
and semiclassical Boltzmann transport theory by considering both the electron
and phonon transport. The electronic part is performed using a modified Becke
and Johnson (mBJ) exchange potential, including spin-orbit coupling (SOC),
while the phonon part is computed using generalized gradient approximation
(GGA). It is found that SOC has remarkable detrimental effect on n-type power
factor, while has a negligible influence in p-type doping, which can be
explained by considering SOC effect on conduction and valence bands. Calculated
results show exceptionally low lattice thermal conductivities in
$\mathrm{CsSnI_3}$ and $\mathrm{CsPbI_3}$, and the corresponding
room-temperature lattice thermal conductivity is 0.54 $\mathrm{W m^{-1}
K^{-1}}$ and 0.25 $\mathrm{W m^{-1} K^{-1}}$. At 1000 K, the maximal figure of
merit $ZT$ is up to 0.63 and 0.64 for $\mathrm{CsSnI_3}$ and $\mathrm{CsPbI_3}$
with scattering time $\tau$=$10^{-14}$ s, and the peak $ZT$ is 0.49 and 0.41
with $\tau$=$10^{-15}$ s. These results make us believe that $\mathrm{CsMI_3}$
(M=Sn and Pb) in perovskite structures may be potential thermoelectric
materials.

###Maximization of the thermoelectric cooling of graded Peltier by analytical heat equation resolution|E. Thiébaut,C. Goupil,F. Pesty,Y. D'Angelo,G. Guegan,P. Lecoeur###

Maximization of the thermoelectric cooling of graded Peltier by analytical heat equation resolution. Increasing the maximum cooling effect of a Peltier cooler can be achieved
through materials and device design. The use of inhomogeneous, FGM
(functionally graded materials) may be adopted in order to increase maximum
cooling without improvement of the zT (figure of merit), however these systems
are usually based on the assumption that the local optimization of the zT is
the suitable criterion to increase thermoelectric performances. In the present
paper, we solved the heat equation in a graded material and performed both
analytic and numerical analysis of a graded Peltier cooler. We find a local
criterion that we used to assess the possible improvement of graded materials
for thermoelectric cooling. A fair improvement of cooling effect is predicted
for semiconductor materials (up to $36\%$) and the best graded system for
cooling is described. The influence of the equation of state of the electronic
gas of the material is discussed, and the difference in term of entropy
production between the graded and the classical system is also described.

###Thermoelectric properties of finite graphene antidot lattices|Tue Gunst,Troels Markussen,Antti-Pekka Jauho,Mads Brandbyge###

Thermoelectric properties of finite graphene antidot lattices. We present calculations of the electronic and thermal transport properties of
graphene antidot lattices with a finite length along the transport direction.
The calculations are based on a single orbital tight-binding model and the
Brenner potential. We show that both electronic and thermal transport
properties converge fast toward the bulk limit with increasing length of the
lattice: only a few repetitions (~6) of the fundamental unit cell are required
to recover the electronic band gap of the infinite lattice as a transport gap
for the finite lattice. We investigate how different antidot shapes and sizes
affect the thermoelectric properties. The resulting thermoelectric figure of
merit, ZT, can exceed 0.25, and it is highly sensitive to the atomic
arrangement of the antidot edges. Specifically, hexagonal holes with pure
zigzag edges lead to an order-of-magnitude smaller ZT as compared to pure
armchair edges. We explain this behavior as a consequence of the localization
of states, which predominantly occurs for zigzag edges, and of an increased
splitting of the electronic minibands, which reduces the power factor.

###Ab initio optimization of phonon drag effect for lower-temperature thermoelectric energy conversion|Jiawei Zhou,Bolin Liao,Bo Qiu,Samuel Huberman,Keivan Esfarjani,Mildred S. Dresselhaus,Gang Chen###

Ab initio optimization of phonon drag effect for lower-temperature thermoelectric energy conversion. While the thermoelectric figure of merit zT above 300K has seen significant
improvement recently, the progress at lower temperatures has been slow, mainly
limited by the relatively low Seebeck coefficient and high thermal
conductivity. Here we report, for the first time, success in first-principles
computation of the phonon drag effect - a coupling phenomenon between electrons
and non-equilibrium phonons - in heavily doped region and its optimization to
enhance the Seebeck coefficient while reducing the phonon thermal conductivity
by nanostructuring. Our simulation quantitatively identifies the major phonons
contributing to the phonon drag, which are spectrally distinct from those
carrying heat, and further reveals that while the phonon drag is reduced in
heavily-doped samples, a significant contribution to Seebeck coefficient still
exists. An ideal phonon filter is proposed to enhance zT of silicon at room
temperature by a factor of 20 to around 0.25, and the enhancement can reach 70
times at 100K. This work opens up a new venue towards better thermoelectrics by
harnessing non-equilibrium phonons.

###Thermoelectric properties of Ba-Cu-Si clathrates|X. Yan,M. X. Chen,S. Laumann,E. Bauer,P. Rogl,R. Podloucky,S. Paschen###

Thermoelectric properties of Ba-Cu-Si clathrates. Thermoelectric properties of the type-I clathrates Ba$_8$Cu$_x$Si$_{46-x}$
($3.6 \leq x \leq 7$, $x$ = nominal Cu content) are investigated both
experimentally and theoretically. The polycrystalline samples are prepared
either by melting, ball milling and hot pressing or by melt spinning, hand
milling and hot pressing techniques. Temperature-dependent electrical
resistivity, $\rho(T)$, and the Seebeck coefficient, $S(T)$, measurements
reveal metal-like behavior for all samples. For $x = 5$ and 6, density
functional theory calculations are performed for deriving the enthalpy of
formation and the electronic structure which is exploited for the calculation
of Seebeck coefficients and conductivity within Boltzmann's transport theory.
For simulating the properties of doped clathrates the rigid band model is
applied. On the basis of the density functional theory results the
experimentally observed compositional dependence of $\rho(T)$ and $S(T)$ of the
whole sample series is analyzed. The highest dimensionless thermoelectric
figure of merit $ZT$ of 0.28 is reached for a melt-spun sample at
$600^{\circ}$C. The relatively low $ZT$ values in this system are attributed to
the too high charge carrier concentrations.

###Thermoelectric Properties of (1-x)LaCoO$_{3.x}$La$_{0.7}$Sr$_{0.3}$MnO$_3$ Composite|Ashutosh Kumar,Karuna Kumari,B. Jayachandran,D. Sivaprahasam,Ajay D Thakur###

Thermoelectric Properties of (1-x)LaCoO$_{3.x}$La$_{0.7}$Sr$_{0.3}$MnO$_3$ Composite. We report the thermoelectric (TE) properties of (1-x)LaCoO3.xLa0.7Sr0.3MnO3
(0 < x < 0.10) composite in a temperature range 320-800 K. Addition of
La0.7Sr0.3MnO3 to LaCoO3 in small amount (5 weight %) improves the overall
Seebeck coefficient ({\alpha}) at higher temperatures. The electrical
conductivity, however, decreases due to a decrease in carrier concentration of
the composite. The decrease in electrical conductivity of the composite at high
temperature may be attributed to the insulating nature of the LSMO above room
temperature. Thermal conductivity (\k{appa}) of all the samples increases with
an increase in the temperature but decreases with increasing LSMO content. We
also report the local variation of the Seebeck coefficient across the composite
samples measured using a precision Seebeck measurement system. A maximum value
of 0.09 for the figure of merit (ZT) is obtained for
0.95LaCoO3.0.05La0.7Sr0.3MnO3 at 620 K which is significantly higher than the
ZT of either of LaCoO3 or La0.7Sr0.3MnO3 at 620 K. This suggests the potential
for enhancement of operating temperatures of hitherto well known
low-temperature thermoelectric materials through suitable compositing approach.

###Thermoelectric Penta-Silicene with a High Room-Temperature Figure of Merit|Zhibin Gao,Jian-Sheng Wang###

Thermoelectric Penta-Silicene with a High Room-Temperature Figure of Merit. Silicon is one of the most frequently used chemical elements of the periodic
table in nanotechnology. Two-dimensional (2D) silicene, a silicon analog of
graphene, has been readily obtained to make field-effect transistors since
2015. Recently, as new members of the silicene family, penta-silicene and its
nanoribbon have been experimentally grown on Ag(110) surface with exotic
electronic properties. However, the thermoelectric performance of
penta-silicene has not been so far studied that would hinder its potential
applications of electric generation from waste heat and solid-state Peltier
coolers. Based on the Boltzmann transport theory and ab initio calculations, we
find that penta-silicene shows a remarkable room temperature figure of merit ZT
of 3.4 and 3.0 at the reachable hole and electron concentrations, respectively.
We attribute this high ZT to the superior pudding-mold electronic band
structure and ultralow lattice thermal conductivity. The discovery provides new
insight into the transport property of pentagonal nanostructures and highlights
the potential applications of thermoelectric materials at room temperature.

###Synthesis, processing and transport properties of cubic and trigonal solid solutions in the Mg-Si-Sn system|Pathikumar Sellappan,Anthony Y. Fong,Masayuki Murata,Yasuhiro Kodera,Javier E. Garay###

Synthesis, processing and transport properties of cubic and trigonal solid solutions in the Mg-Si-Sn system. Mg-Si-Sn solid solutions have interesting and useful semiconducting
properties particularly for thermoelectric applications. While cubic solid
solutions have been prepared by a variety of methods, solid solutions with a
trigonal crystal structure have not been reported. We employed a combination of
high energy ball milling and current activated pressure assisted densification
(CAPAD) to induce and complete phase transformation from cubic to trigonal
phase, forming dense trigonal composites. This is the first successful
preparation of the trigonal phase in Mg-Si-Sn stem. In addition, we report the
transport properties of both the cubic and trigonal composites measured in the
160 to 400 K range. The electrical conductivity of the trigonal composites was
several times higher compared to the cubic counterparts while thermal
conductivity is lower but in the range of previously reported Mg-Si-Sn
materials. The Seebeck coefficient of the trigonal composites is lower compared
to the cubic samples, leading to low ZT values. The ZT of the cubic samples,
however is higher than previously reported for un-doped Mg-Si-Sn solid
solutions which is promising for thermoelectric applications.

###Two-valence band electron and heat transport in monocrystalline PbTe-CdTe solid solutions with high Cd content|M. Szot,P. Pfeffer,K. Dybko,A. Szczerbakow,L. Kowalczyk,P. Dziawa,R. Minikayev,T. Zayarnyuk,K. Piotrowski,M. U. Gutowska,A. Szewczyk,T. Story,W. Zawadzki###

Two-valence band electron and heat transport in monocrystalline PbTe-CdTe solid solutions with high Cd content. High quality p-type PbTe-CdTe monocrystalline alloys containing up to 10
at.$\%$ of Cd are obtained by self-selecting vapor transport method. Mid
infrared photoluminescence experiments are performed to follow the variation of
the fundamental energy gap as a function of Cd content. The Hall mobility,
thermoelectric power, thermal conductivity and thermoelectric figure of merit
parameter $ZT$ are investigated experimentally and theoretically paying
particular attention to the two-valence band structure of the material. It is
shown that the heavy-hole band near the $\Sigma$ point of the Brillouin zone
plays an important role and is responsible for the Pb$_{1-x}$Cd$_x$Te hole
transport at higher Cd-content. Our data and their description can serve as the
standard for Pb$_{1-x}$Cd$_x$Te single crystals with $x$ up to 0.1. It is
shown, that monocrystalline Pb$_{1-x}$Cd$_x$Te samples with relatively low Cd
content of about 1 at.\% and hole concentration of the order of 10$^{20}$
cm$^{-3}$ may exhibit $ZT \approx$ 1.4 at 600 K.

###Achieving high figure-of-merit in Nb-doped Na$_{0.74}$CoO$_{2}$ compound at high temperature region|Arzena Khatun,Shamim Sk,Jayashree Pati,R. S. Dhaka,Sudhir K. Pandey###

Achieving high figure-of-merit in Nb-doped Na$_{0.74}$CoO$_{2}$ compound at high temperature region. We report the thermoelectric (TE) properties of
Na$_{0.74}$Co$_{0.95}$Nb$_{0.05}$O$_{2}$ in the temperature range $300-1200$ K,
as a potential candidate for p-type thermoelectric material. The experimental
values of Seebeck coefficient (S) are $ \sim $ $82-121$ $ \mu $V/K measured in
the temperature range $300-620$ K. The positive values of S in the entire
temperature range indicates p-type behaviour of the compound. At 300 K the
experimental value of thermal conductivity ($ \kappa $) is $ \sim $ 1.88 W/m-K
that increases up to $ \sim $ 420 K, then decreases till 620 K with
corresponding value $ \sim $ 1.86 W/m-K. To understand the experimentally
observed transport properties, we have calculated S and $\rho$ of this
compound. Then, based on theoretical understanding, we have estimated
\textit{figure-of-merit} (ZT) up to 1200 K by using calculated S and $\rho$
values with extrapolated experimental $\kappa$. The value of ZT is found to be
$\sim$ 0.03 at 300 K, whereas, the highest value is observed as $\sim$ 1.7 at
1200 K. Finally, we have calculated the efficiency ($\eta$) by keeping the cold
end temperature (T$_{c}$) fixed at 500 K and varying hot end temperature
(T$_{h}$) from 500 to 1200 K, respectively. The maximum value of $\eta$ is
found to be $\sim$ 8 %, when T$_{c}$ and T$_{h}$ are fixed at 500 and 1200 K,
respectively. This result suggests that
Na$_{0.74}$Co$_{0.95}$Nb$_{0.05}$O$_{2}$ compound can be used as a p-leg for
making high temperature TE generator (TEG).

###Thermoelectric figure of merit enhancement in dissipative superlattice structures|Pankaj Priyadarshi,Bhaskaran Muralidharan###

Thermoelectric figure of merit enhancement in dissipative superlattice structures. Utilizing the non-coherent quantum transport formalism, we investigate
thermoelectric performance across dissipative superlattice configurations in
the linear regime of operation. Using the {\it{dissipative}} non-equilibrium
Green's function formalism coupled self-consistently with the Poisson's
equation, we report an enhanced figure of merit $zT$ in the multi-barrier
device designs. The proposed enhancement, we show, is a result of a drastic
reduction in the electronic thermal conductance triggered via non-coherent
transport. We show that a maximum $zT$ value of 18 can be achieved via the
inclusion of non-coherent elastic scattering processes. There is also a
reasonable enhancement in the Seebeck coefficient, with a maximum of $1000~\mu
V/K$, which we attribute to an enhancement in electronic filtering arising from
the non-coherent transport. Distinctly the thermal conduction is drastically
reduced as the length of the superlattice scales up, although the power factor
shows an overall degradation. While the presence of interfaces is known to kill
phonon thermal conduction, our analysis shows that non-coherent processes in
superlattice structures can effectively kill electronic thermal conduction
also. We believe that the analysis presented here could set the stage to
understand better the interplay between non-coherent scattering and coherent
quantum processes in the electronic engineering of heterostructure
thermoelectric devices.

###First-principles prediction of Structural Stability and Thermoelectric Properties of SrGaSnH|Enamul Haque,Mizanur Rahaman###

First-principles prediction of Structural Stability and Thermoelectric Properties of SrGaSnH. Thermoelectric materials based on earth-abundant and non-toxic elements are
very useful in cost-effective and eco-friendly waste heat management systems.
The constituents of SrGaSnH are earth-abundant and non-toxic, thus we have
chosen SrSnGaH to study its structural stability and thermoelectric properties
by using DFT, DFPT, and semi-classical Boltzmann transport theory. Our elastic
and phonons calculations show that the compound has good structural stability.
The electronic structure calculation discloses that it is an indirect bandgap
(0.63 eV by mBJ+SOC) semiconductor. Light band hole effective mass leads to
higher electrical conductivity along x-axis than that of along z-axis. On the
other side, the weak phonon scattering leads to high lattice thermal
conductivity ~10.5 W m-1K-1 at 300 K. Although the power factor (PF) is very
high along the x-axis (above 10 mW m-1K-2 at 300 K), such large kl dramatically
reduces ZT. The maximum values of in-plane and cross-plane ZT are ~1 (n-type),
0.8 (p-type) and 0.6 (n-type), (0.2 p-type) at 700 K, respectively. The present
study has revealed that this compound has strong potential in eco-friendly TE
applications.

###Effect of interface resistance on thermoelectric properties in (1-x)La$_{0.95}$Sr$_{0.05}$Co$_{0.95}$Mn$_{0.05}$O$_3$/(x)WC composite|Ashutosh Kumar,Krzysztof T. Wojciechowski###

Effect of interface resistance on thermoelectric properties in (1-x)La$_{0.95}$Sr$_{0.05}$Co$_{0.95}$Mn$_{0.05}$O$_3$/(x)WC composite. In this study, the synergistic effect of the particle size of the dispersed
phase and the interface thermal resistance (R$_{int}$) between the phases on
the phonon thermal conductivity ($\kappa_{ph}$) of the
(1-x)La$_{0.95}$Sr$_{0.05}$Co$_{0.95}$Mn$_{0.05}$O$_3$/(x)WC thermoelectric
composite, is demonstrated. Further, the correlation between the R$_{int}$ and
the Kapitza radius is discussed using the Bruggeman's asymmetrical model. In
particular, the polycrystalline
La$_{0.95}$Sr$_{0.05}$Co$_{0.95}$Mn$_{0.05}$O$_3$ sample is synthesized using a
standard-solid state route. The presence of WC nanoparticle is confirmed from
the electron microscopy images. Electrical conductivity ($\sigma$) increases,
and the Seebeck coefficient ($\alpha$) decreases with the increase in
conducting WC volume fraction in the composite. The simultaneous increase in
$\sigma$ and a decrease in $\kappa_{ph}$ with the WC volume fraction results in
an increased figure of merit (zT) for
(1-x)La$_{0.95}$Sr$_{0.05}$Co$_{0.95}$Mn$_{0.05}$O$_3$/(x)WC composite. A
maximum zT $\sim$ 0.20 is obtained for
(1-x)La$_{0.95}$Sr$_{0.05}$Co$_{0.95}$Mn$_{0.05}$O$_3$/(x)WC composite for
x=0.010 at 463 K. The results obtained in the present study shows promise to
design thermoelectric composites with desired phonon thermal conductivity
considering the elastic properties between the phases.

###Dynamic doping and Cottrell atmosphere optimize the thermoelectric performance of n-type PbTe|Yuan Yu,Chongjian Zhou,Xiangzhao Zhang,Lamya Abdellaoui,Christian Doberstein,Benjamin Berkels,Bangzhi Ge,Guanjun Qiao,Christina Scheu,Matthias Wuttig,Oana Cojocaru-Mirédin,Siyuan Zhang###

Dynamic doping and Cottrell atmosphere optimize the thermoelectric performance of n-type PbTe. High thermoelectric energy conversion efficiency requires a large
figure-of-merit, zT, over a broad temperature range. To achieve this, we
optimize the carrier concentrations of n-type PbTe from room up to hot-end
temperatures by co-doping Bi and Ag. Bi is an efficient n-type dopant in PbTe,
often leading to excessive carrier concentration at room temperature. As
revealed by density functional theory calculations, the formation of Bi and Ag
defect complexes is exploited to optimize the room temperature carrier
concentration. At elevated temperatures, we demonstrate the dynamic dissolution
of Ag2Te precipitates in PbTe in situ by heating in a scanning transmission
electron microscope. The release of n-type Ag interstitials with increasing
temperature fulfills the requirement of higher carrier concentrations at the
hot end. Moreover, as characterized by atom probe tomography, Ag atoms
aggregate along parallel dislocation arrays to form Cottrell atmospheres. This
results in enhanced phonon scattering and leads to a low lattice thermal
conductivity. As a result of the synergy of dynamic doping and phonon
scattering at decorated dislocations, an average zT of 1.0 is achieved in
n-type Bi/Ag-codoped PbTe between 400 and 825 K. Introducing dopants with
temperature-dependent solubility and strong interaction with dislocation cores
enables simultaneous optimization of the average power factor and thermal
conductivity, providing a new concept to exploit in the field of
thermoelectrics.

###Thermoelectric properties of high-entropy rare-earth cobaltates|Ashutosh Kumar,Diana Dragoe,David Bérardan,Nita Dragoe###

Thermoelectric properties of high-entropy rare-earth cobaltates. High-entropy concept introduced with a promising paradigm to obtain exotic
physical properties has motivated us to explore the thermoelectric properties
of Sr-substituted high-entropy rare-earth cobaltates i.e.,
(LaNdPrSmEu)$_{1-x}$Sr$_x$CoO3 (0 \leq x \leq 0.10). The structural analysis of
the samples synthesized using the standard solid-state route, confirms the
orthorhombic structure with the Pbnm space group. The Seebeck coefficient and
electrical resistivity decrease with rising Sr concentration as well as with an
increase in temperature. The multiple A-site ions in high-entropy rare-earth
cobaltates result in an improved Seebeck coefficient ({\alpha}) compared to
La$_{0.95}$Sr$_{0.05}$CoO$_3$, associated with a decrease in the Co-O-Co bond
angle, which further enhances the power factor. The random distribution of
cations at the rare-earth site results in a significant lowering of phonon
thermal conductivity. As a result, a maximum figure of merit (zT) of 0.23 is
obtained at 350K for (LaNdPrSmEu)$_{0.95}$Sr$_{0.05}$CoO$_3$, which is one of
the highest values of zT reported at this temperature for oxide materials. This
study shows promise to decouple thermoelectric parameters using the
high-entropy concept in several materials.

###Tuning Phononic and Electronic Contributions of Thermoelectric in defected S-Shape Graphene Nanoribbons|M. Amir Bazrafshan,Farhad Khoeini###

Tuning Phononic and Electronic Contributions of Thermoelectric in defected S-Shape Graphene Nanoribbons. Thermoelectrics as a way to use waste heat, is essential in electronic
industries, but its low performance at operational temperatures makes it
inappropriate in practical applications. Tailoring graphene can change its
properties. In this work, we are interested in studying the transport
properties of S-shape graphene structures with the single vacancy (SV) and
double vacancy (DV) models. The structures are composed of a chiral part, which
is an armchair graphene nanoribbon, and two zigzag graphene ribbons. We
investigate the changes in the figure of merit by means of the Seebeck
coefficient, electronic conductance, and electronic and phononic conductances
with the vacancies in different device sizes. The transport properties of the
system are studied by using the non-equilibrium Greens function method, so that
the related Hamiltonians (dynamical matrices) are obtained from the
tight-binding (force constant) model. The maximum figure of merit (ZT) obtains
for the DVs in all lengths. Physical properties of such a system can be tuned
by controlling various parameters such as the location and the type of the
defects, and the device size. Our findings show that lengthening the structure
can reduce phononic contribution, and single vacancies than double vacancies
can better distinguish between electronic thermal conductance behavior and
electronic conductance one. Namely, vacancy engineering can significantly
increase thermoelectric performance. In the large devices, the SVs can increase
the ZT up to 2.5 times.

###Effect of crystal field engineering and Fermi level optimization on thermoelectric properties of Ge$_{1.01}$Te: Experimental investigation and theoretical insight|Ashutosh Kumar,Preeti Bhumla,D. Sivaprahasam,Saswata Bhattacharya,Nita Dragoe###

Effect of crystal field engineering and Fermi level optimization on thermoelectric properties of Ge$_{1.01}$Te: Experimental investigation and theoretical insight. This study shows a method of enhancing the thermoelectric properties of
GeTe-based materials by Ti and Bi co-doping on cation sites along with
self-doping with Ge via simultaneous optimization of electronic (via crystal
field engineering, and precise Fermi level optimization) and thermal (via
point-defect scattering) transport properties. The pristine GeTe possesses high
carrier concentration ($n$) due to intrinsic Ge vacancies, low Seebeck
coefficient ($\alpha$), and high thermal conductivity ($\kappa$). The Ge
vacancy optimization and crystal field engineering results in an enhanced
$\alpha$ via excess Ge and Ti doping, which is further improved by band
structure engineering through Bi doping. As a result of improved $\alpha$ and
optimized Fermi level (carrier concentration), an enhanced power factor
($\alpha^2\sigma$) is obtained for Ti--Bi co-doped Ge$_{1.01}$Te. These
experimental results are also evidenced by theoretical calculations of band
structure, and thermoelectric parameters using density functional theory and
Boltztrap calculations, respectively. A significant reduction in the phonon
thermal conductivity ($\kappa_{ph}$) from $\sim$ 3.5 W.m$^{-1}$.K$^{-1}$ to
$\sim$ 1.06 W.m$^{-1}$.K$^{-1}$ at 300\,K for Ti--Bi co-doping in GeTe,
attributed to point-defect scattering due to mass and strain field fluctuation,
in line with the Debye-Callaway model. The phonon dispersion calculations show
a decreasing group velocity in Ti--Bi co-doped GeTe, supporting the obtained
reduced $\kappa_{ph}$. The strategies used in the present study can
significantly increase the effective mass, optimize the carrier concentration,
and decrease phonon thermal conductivity while achieving an impressive maximum
zT value of 1.75 at 773\,K and average zT (zT$_{av}$) of 1.03 for
Ge$_{0.91}$Ti$_{0.02}$Bi$_{0.08}$Te over a temperature range of 300-773\,K.

###Two functionals approach in DFT for the prediction of thermoelectric properties of Fe$_{2}$ScX (X = P, As, Sb) full Heusler compounds|Shivprasad S. Shastri,Sudhir K. Pandey###

Two functionals approach in DFT for the prediction of thermoelectric properties of Fe$_{2}$ScX (X = P, As, Sb) full Heusler compounds. In the quest of new thermoelectric (TE) materials with high power factors,
full-Heusler compounds having flat band are found to be promising candidates.
In this direction, Fe$_{2}$ScX (X=P,As,Sb) compounds are investigated using mBJ
for the band gap and SCAN to describe the electronic bands and phonon
properties for TE applications. The band gaps obtained from mBJ are 0.81 eV,
0.69 eV and 0.60 eV for Fe$_{2}$ScX compounds, respectively. The phonon
dispersion, phonon density of states (DOS) and partial DOS are calculated. The
phonon contributions to specific heat are obtained as a function of temperature
under harmonic approximation. The electronic band structutre calculated from
mBJ and SCAN functionals are qualitatively compared. The TE parameters are
calculated for both hole and electron dopings under semiclassical theory. We
use simple, but reasonable method to estimate phonon relaxation time
($\tau_{ph}$). Using the specific heat, estimated $\tau_{ph}$ and slopes (phase
velocity) of acoustic branches in the linear region, lattice thermal
conductivity ($\kappa_{ph}$) at 300 K is calculated for three compounds. The
obtained values of $\kappa_{ph}$ with constant $\tau_{ph}$ are 18.2, 13.6 and
10.3 $Wm^{-1}K^{-1}$, respectively. Finally, the temperature dependent figure
of merit $ZT$ values are calculated for optimal carrier concentrations in the
doping range considered, to evaluate the materials for TE application. The $ZT$
values for n-type Fe$_{2}$ScX, in 900-1200 K, are 0.34-0.43, 0.40-0.48 and
0.45-0.52, respectively. While, the p-type Fe$_{2}$ScX have $ZT$ of 0.25-0.34,
0.20-0.28 and 0.18-0.26, respectively in the same temperature range. The $ZT$
values suggest that, Fe$_{2}$ScX compounds can be promising materials in high
temperature power generation application on successful synthesis and further
$\kappa_{ph}$ reduction by methods like nanostructuring.

###Numerical study of the thermoelectric power factor in ultra-thin Si nanowires|Neophytos Neophytou,Hans Kosina###

Numerical study of the thermoelectric power factor in ultra-thin Si nanowires. Low dimensional structures have demonstrated improved thermoelectric (TE)
performance because of a drastic reduction in their thermal conductivity,
{\kappa}l. This has been observed for a variety of materials, even for
traditionally poor thermoelectrics such as silicon. Other than the reduction in
{\kappa}l, further improvements in the TE figure of merit ZT could potentially
originate from the thermoelectric power factor. In this work, we couple the
ballistic (Landauer) and diffusive linearized Boltzmann electron transport
theory to the atomistic sp3d5s*-spin-orbit-coupled tight-binding (TB)
electronic structure model. We calculate the room temperature electrical
conductivity, Seebeck coefficient, and power factor of narrow 1D Si nanowires
(NWs). We describe the numerical formulation of coupling TB to those transport
formalisms, the approximations involved, and explain the differences in the
conclusions obtained from each model. We investigate the effects of cross
section size, transport orientation and confinement orientation, and the
influence of the different scattering mechanisms. We show that such methodology
can provide robust results for structures including thousands of atoms in the
simulation domain and extending to length scales beyond 10nm, and point towards
insightful design directions using the length scale and geometry as a design
degree of freedom. We find that the effect of low dimensionality on the
thermoelectric power factor of Si NWs can be observed at diameters below ~7nm,
and that quantum confinement and different transport orientations offer the
possibility for power factor optimization.

###Physical properties of the thermoelectric cubic lanthanum chalcogenides La3-yX4 (X=S,Se,Te) from first-principles|Romain Viennois,Kinga Niedziolka,Philippe Jund###

Physical properties of the thermoelectric cubic lanthanum chalcogenides La3-yX4 (X=S,Se,Te) from first-principles. We report ab-initio calculations of the stability, lattice dynamics,
electronic and thermoelectric properties of cubic La3-yX4 (X=S,Se,Te) materials
in view of analyzing their potential for thermoelectric applications. The
lanthanum motions are strongly coupled to the tellurium motions in the
telluride, whereas the motions of both types of atoms are decoupled in the
sulfides. Nevertheless, this has no impact on their thermal properties because
experimentally all compounds have low thermal conductivity. We believe that
this is due to Umklapp scattering of the acoustical modes, notably by the low
energy optical modes at about 7-8 meV found in all three chalcogenides, as in
cage compounds such as skutterudites or clathrates, even though there are no
cages in the cubic Th3P4 structure. We find that the energy bandgap increases
from the telluride to the sulfide in good agreement with the experiments.
However, due to their similar band structure, we find that all three compounds
have almost identical thermoelectric properties. Our results agree
qualitatively with the experiments, especially in the case of the telluride for
which a great amount of data exists. All our results indicate that the sulfides
have strong potential for thermoelectricity and could replace the tellurides if
the charge carrier concentration is optimized. Finally, we predict also a
larger maximum ZT for the p-type doped materials than for the n-type doped
ones, even though compounds with p-doping have still to be synthesized. Thus
our results indicate the possibility to make high temperature performing
thermo-generators based only on La3X4 compounds.

###Pd$_{2}$Se$_{3}$ Monolayer: A Promising Two Dimensional Thermoelectric Material with Ultralow Lattice Thermal Conductivity and High Power Factor|S. Shahab Naghavi,Jiangang He,Yi Xia,C. Wolverton###

Pd$_{2}$Se$_{3}$ Monolayer: A Promising Two Dimensional Thermoelectric Material with Ultralow Lattice Thermal Conductivity and High Power Factor. A high power factor and low lattice thermal conductivity are two essential
ingredients of highly efficient thermoelectric materials. Although monolayers
of transition metal dichalcogenides possess high power factors, high lattice
thermal conductivities significantly impede their practical applications. Our
first-principles calculations show that these two ingredients are well
fulfilled in the recently synthesized Pd$_{2}$Se$_{3}$ monolayer, whose crystal
structure is composed of [Se$_{2}$]$^{2-}$ dimers, Se$^{2-}$ anions, and
Pd$^{2+}$ cations coordinated in a square planar manner. Our detailed analysis
of third-order interatomic force constants reveals that the anharmonicity and
soft phonon modes associated with [Se$_2$]$^{2-}$ dimers lead to ultra-low
lattice thermal conductivities in Pd$_{2}$Se$_{3}$ monolayers (1.5 and 2.9
Wm$^{-1}$K$^{-1}$ along the $a$- and $b$-axes at 300\,K respectively), which
are comparable to those of high-performance bulk thermoelectric materials such
as PbTe. Moreover, the "pudding-mold" type band structure, caused by Pd$^{2+}$
($d^{8}$) cations coordinated in a square planar crystal field, leads to high
power factors in Pd$_{2}$Se$_{3}$ monolayers. Consequently, both electron and
hole doped thermoelectric materials with a considerably high $zT$ can be
achieved at moderate carrier concentrations, suggesting that Pd$_{2}$Se$_{3}$
is a promising two-dimensional thermoelectric material.

###Bulk Nanocrystalline Thermoelectrics Based on Bi-Sb-Te Solid Solution|L. P. Bulat.,D. A. Pshenai-Severin,V. V. Karatayev,V. B. Osvenskii,Yu. N. Parkhomenko,V. Lavrentev,A. Sorokin,V. D. Blank,G. I. Pivovarov,V. T. Bublik,N. Yu. Tabachkova###

Bulk Nanocrystalline Thermoelectrics Based on Bi-Sb-Te Solid Solution. A nanopowder from p-Bi-Sb-Te with particles ~ 10 nm were fabricated by the
ball milling using different technological modes. Cold and hot pressing at
different conditions and also SPS process were used for consolidation of the
powder into a bulk nanostructure and nanocomposites. The main factors allowing
slowing-down of the growth of nanograins as a result of recrystallization are
the reduction of the temperature and of the duration of the pressing, the
increase of the pressure, as well as addition of small value additives (like
MoS2, thermally expanded graphite or fullerenes). It was reached the
thermoelectric figure of merit ZT=1.22 (at 360 K) in the bulk nanostructure
Bi0,4Sb1,6Te3 fabricated by SPS method. Some mechanisms of the improvement of
the thermoelectric efficiency in bulk nanocrystalline semiconductors based on
BixSb2-xTe3 are studied theoretically. The reduction of nanograin size can lead
to improvement of the thermoelectric figure of merit. The theoretical
dependence of the electric and heat conductivities and the thermoelectric power
as the function of nanograins size in BixSb2-xTe3 bulk nanostructure are quite
accurately correlates with the experimental data.

###Surface oxidation and thermoelectric properties of indium-doped tin telluride nanowires|Z. Li,E. Z. Xu,Y. Losovyj,N. Li,A. P. Chen,B. Swartzentruber,N. Sinitsyn,J. K. Yoo,Q. X. Jia,S. X. Zhang###

Surface oxidation and thermoelectric properties of indium-doped tin telluride nanowires. The recent discovery of excellent thermoelectric properties and topological
surface states in SnTe-based compounds has attracted extensive attention in
various research areas. Indium doped SnTe is of particular interest because,
depending on the doping level, it can either generate resonant states in the
bulk valence band leading to enhanced thermoelectric properties, or induce
superconductivity that coexists with topological states. Here we report on the
vapor deposition of In-doped SnTe nanowires and the study of their surface
oxidation and thermoelectric properties. The nanowire growth is assisted by Au
catalysts, and their morphologies vary as a function of substrate position and
temperature. Transmission electron microscopy characterization reveals the
formation of amorphous surface in single crystalline nanowires. X-ray
photoelectron spectroscopy studies suggest that the nanowire surface is
composed of In2O3, SnO2, Te and TeO2 which can be readily removed by argon ion
sputtering. Exposure of the cleaned nanowires to atmosphere yields rapid
oxidation of the surface within only one minute. Characterizations of
electrical conductivity {\sigma}, thermopower S, and thermal conductivity
\k{appa} were performed on the same In-doped nanowire which shows suppressed
{\sigma} and \k{appa} but enhanced S yielding an improved thermoelectric figure
of merit ZT than the undoped SnTe.

###High thermoelectric efficiency in monolayer PbI$_2$ from 300 K to 900 K|Bo Peng,Haodong Mei,Hao Zhang,Hezhu Shao,Ke Xu,Gang Ni,Qingyuan Jin,Costas M. Soukoulis,Heyuan Zhu###

High thermoelectric efficiency in monolayer PbI$_2$ from 300 K to 900 K. By using a first-principles approach, monolayer PbI$_2$ is found to have
great potential in thermoelectric applications. The linear Boltzmann transport
equation is applied to obtain the perturbation to the electron distribution by
different scattering mechanisms. The mobility is mainly limited by the
deformation-potential interaction with long-wavelength acoustic vibrations at
low carrier concentrations. At high concentrations, ionized impurity scattering
becomes stronger. The electrical conductivity and Seebeck coefficient are
calculated accurately over various ranges of temperature and carrier
concentration. The lattice thermal conductivity of PbI$_2$, 0.065 W/mK at 300
K, is the lowest among other 2D thermoelectric materials. Such ultralow thermal
conductivity is attributed to large atomic mass, weak interatomic bonding,
strong anharmonicity, and localized vibrations in which the vast majority of
heat is trapped. These electrical and phonon transport properties enable high
thermoelectric figure of merit over 1 for both p-type and n-type doping from
300 K to 900 K. A maximum $zT$ of 4.9 is achieved at 900 K with an electron
concentration of 1.9$\times$10$^{12}$ cm$^{-2}$. Our work shows exceptionally
good thermoelectric energy conversion efficiency in monolayer PbI$_2$, which
can be integrated to the existing photovoltaic devices.

###Signature of the transition to a bound state in thermoelectric quantum transport|Étienne Jussiau,Masahiro Hasegawa,Robert S. Whitney###

Signature of the transition to a bound state in thermoelectric quantum transport. We study a quantum dot coupled to two semiconducting reservoirs, when the dot
level and the electrochemical potential are both close to a band edge in the
reservoirs. This is modelled with an exactly solvable Hamiltonian without
interactions (the Fano-Anderson model). The model is known to show an abrupt
transition as the dot-reservoir coupling is increased into the strong-coupling
regime for a broad class of band structures. This transition involves an
infinite-lifetime bound state appearing in the band gap. We find a signature of
this transition in the continuum states of the model, visible as a
discontinuous behaviour of the dot's transmission function. This can result in
the steady-state DC electric and thermoelectric responses having a very strong
dependence on coupling close to critical coupling. We give examples where the
conductances and the thermoelectric power factor exhibit huge peaks at critical
coupling, while the thermoelectric figure of merit ZT grows as the coupling
approaches critical coupling, with a small dip at critical coupling. The
critical coupling is thus a sweet spot for such thermoelectric devices, as the
power output is maximal at this point without a significant change of
efficiency.

###Half-Heusler Compounds: Promising Materials For Mid-To-High Temperature Thermoelectric Conversion|S. Joseph Poon###

Half-Heusler Compounds: Promising Materials For Mid-To-High Temperature Thermoelectric Conversion. Half-Heusler compounds (space group Fm3m) has garnered increasing attention
in recent years in the thermoelectric community. Three decades ago, refractory
RNiSn half-Heusler compounds (R represents refractory metals such as Hf, Zr,
Ti) were found to be narrow-gap semiconductors with large Seebeck coefficients
in 100s of micro-volt per Kelvin. Today, half-Heusler (HH) compounds have
emerged as promising thermoelectric materials in the intermediate temperature
range (400-800oC). HH materials are endowed with good thermal stability and
scalability. Thermoelectric n-p modules based on HH materials demonstrate
conversion efficiency near 10% and power density output near 9 W/cm2. The
objective of this article is to present a historical account of the research
and development of thermoelectric half-Heusler compounds. Particularly, there
have been notable achievements since 2012 thanks to the emergence of new
approaches. As a result, ZT has risen from ~1 to 1.5. The various advances made
since the early 1990s to the present are recounted by categorizing half-Heusler
materials into three generations (Gen): Gen-1 Gen-2, and Gen-3 HH materials.

###Dimensional crossover and enhanced thermoelectric efficiency due to broken symmetry in graphene antidot lattices|M. Neşet Çınar,Hâldun Sevinçli###

Dimensional crossover and enhanced thermoelectric efficiency due to broken symmetry in graphene antidot lattices. Graphene antidot lattices (GALs) are two-dimensional (2D) monolayers with
periodically placed holes in otherwise pristine graphene. We investigate the
electronic properties of symmetric and asymmetric GAL structures having
hexagonal holes, and show that anisotropic 2D GALs can display a dimensional
crossover such that effectively one-dimensional (1D) electronic structures can
be realized in two-dimensions around the charge neutrality point. We
investigate the transport and thermoelectric properties of these 2D GALs by
using non-equilibrium Green function (NEGF) method. Dimensional crossover
manifests itself as transmission plateaus, a characteristic feature of 1D
systems, and enhancement of thermoelectric efficiency, where thermoelectric
figure of merit, $zT$, can be as high as 0.9 at room temperature. We also study
the transport properties in the presence of Anderson disorder and find that
mean free paths of effectively 1D electrons of anisotropic configuration are
much longer than their isotropic counterparts. We further argue that
dimensional crossover due to broken symmetry and enhancement of thermoelectric
efficiency can be nanostructuring strategy virtually for all 2D materials.

###Cost-Performance Trade-off in Thermoelectric Air Conditioning System with Graded and Constant Material Properties|Abhishek Saini,Sarah J. Watzman,Je-Hyeong Bahk###

Cost-Performance Trade-off in Thermoelectric Air Conditioning System with Graded and Constant Material Properties. Thermoelectric (TE) air cooling is a solid-state technology that has the
potential to replace conventional vapor compression-based air conditioning. In
this paper, we present a detailed system-level modeling for thermoelectric air
conditioning system with position-dependent (graded) and constant material
properties. Strategies for design optimization of the system are provided in
terms of cost-performance trade-off. Realistic convection heat transfer at both
sides of the system are taken into account in our modeling. Effects of
convection heat transfer coefficients, air flowrate, and thermoelectric
material properties are investigated with varying key parameters such as TE leg
thickness, module fill factor, and input current. Both constant material
properties and graded properties are considered for the TE materials, and they
are compared in terms of the degree of cooling, coefficient of performance
(COP), and power consumption. For graded materials, we employ one-dimensional
finite element methods to solve the coupled electrical and thermal current
equations with arbitrary profile of material properties varying with position
along the TE legs. We find that graded materials can enhance the degree of
cooling, but only at the expense of COP, compared to the case of constant
property materials. With constant material properties of ZT = 1 and relatively
low electric current, the power consumption of TE cooler can be lower than
those of conventional air conditioners at an equivalent cooling capacity.
Considering additional advantages such as demand-flexible operation, low noise,
and high scalability, thermoelectric cooling could be a competitive technology
for future air conditioning applications.

###Electronic transport descriptors for the rapid screening of thermoelectric materials|Tianqi Deng,Jose Recatala-Gomez,Masato Ohnishi,D. V. Maheshwar Repaka,Pawan Kumar,Ady Suwardi,Anas Abutaha,Iris Nandhakumar,Kanishka Biswas,Michael B. Sullivan,Gang Wu,Junichiro Shiomi,Shuo-Wang Yang,Kedar Hippalgaonkar###

Electronic transport descriptors for the rapid screening of thermoelectric materials. The discovery of novel materials for thermoelectric energy conversion has
potential to be accelerated by data-driven screening combined with
high-throughput calculations. One way to increase the efficacy of successfully
choosing a candidate material is through its evaluation using transport
descriptors. Using a data-driven screening, we selected 12 potential candidates
in the trigonal ABX2 family, followed by charge transport property simulations
from first principles. The results suggest that carrier scattering processes in
these materials are dominated by ionised impurities and polar optical phonons,
contrary to the oft-assumed acoustic-phonon-dominated scattering. Combined with
calculations of thermal conductivity based on three-phonon scattering, we
predict p-type AgBiS2 and TlBiTe2 as potential high-performance thermoelectrics
in the intermediate temperature range for low grade waste heat harvesting, with
a predicted zT above 1 at 500 K. Using these data, we further derive
ground-state transport descriptors for the carrier mobility and the
thermoelectric power factor. In addition to low carrier mass, high dielectric
constant was found to be an important factor towards high carrier mobility. A
quadratic correlation between dielectric constant and transport performance was
established and further validated with literature. Looking ahead, dielectric
constant can potentially be exploited as an independent tuning knob for
improving the thermoelectric performance.

###Ab-initio investigations for Structural, Mechanical, Optoelectronic, and Thermoelectric properties of Ba2SbXO6 (X=Nb, Ta) compounds|Hansraj,K. C. Bhamu,Sung Gu Kang,A. K. Kushwaha,D. P. Rai,Subrahmanyam Sappati,J. Sahariya,Amit Soni###

Ab-initio investigations for Structural, Mechanical, Optoelectronic, and Thermoelectric properties of Ba2SbXO6 (X=Nb, Ta) compounds. We report the structural, mechanical, electronic, optical, thermoelectric
properties and spectroscopic limited maximum efficiency (SLME) of oxide double
perovskite structure Ba2SbNbO6 and Ba2SbTaO6 compounds. All the investigations
were performed through the first-principles density functional theory (DFT).
The obtained values for the elastic constants reveal the mechanical stability
of the studied compounds. The calculated data of bulk modulus (B), shear
modulus (G), and Young's modulus (E) for Ba2SbTaO6 are found to be greater than
those of Ba2SbNbO6. The ratio of Bulk to shear ratio (B/G) shows that Ba2SbNbO6
and Ba2SbTaO6 are ductile. The computed electronic band structure reveals the
semiconducting nature of both compounds. We have also studied the electron
relaxation time-dependent thermoelectric properties, such as Seebeck
coefficient, thermal conductivity, electrical conductivity, thermoelectric
power factor, and the figure of merit as a function of chemical potential at
various temperatures for p-type and n-type charge carriers. The high absorption
spectra and good figure of merit (ZT) reveal that both the studied compounds,
Ba2SbXO6 (X = Nb, Ta) are promising materials for photovoltaic and
thermoelectric applications. The calculated SLME of 26.8% reveals that Ba2SNbO6
is an appealing candidate for single-junction solar cells.

###Giant reduction of thermal conductivity in twinning superlattice InAsSb nanowires|Lorenzo Peri,Domenic Prete,Valeria Demontis,Valentina Zannier,Francesca Rossi,Lucia Sorba,Fabio Beltram,Francesco Rossella###

Giant reduction of thermal conductivity in twinning superlattice InAsSb nanowires. Semiconductor nanostructures hold great promise for high-efficiency waste
heat recovery exploiting thermoelectric energy conversion, a technological
breakthrough that could significantly contribute to providing environmentally
friendly energy sources as well as in enabling the realization of self-powered
biomedical and wearable devices. A crucial requirement in this field is the
reduction of the thermal conductivity of the thermoelectric material without
detrimentally affecting its electrical transport properties. In this work we
demonstrate a drastic reduction of thermal conductivity in III-V semiconductor
nanowires due to the presence of intentionally realized periodic crystal
lattice twin planes. The electrical and thermal transport of these
nanostructures, known as twinning superlattice nanowires, have been probed and
compared with their twin-free counterparts, showing a one order of magnitude
decrease of thermal conductivity while maintaining unaltered electrical
transport properties, thus yielding a factor ten enhancement of the
thermoelectric figure of merit, ZT. Our study reports for the first time the
experimental measurement of electrical and thermal properties in twinning
superlattice nanowires, which emerge as a novel class of nanomaterials for high
efficiency thermoelectric energy harvesting.

###Thermoelectric properties of cement composite analogues from first principles calculations|Esther Orisakwe,Conrad Johnston,Ruchita Jani,Xiaoli Liu,Lorenzo Stella,Jorge Kohanoff,Niall Holmes,Brian Norton,Ming Qu,Hongxi Yin,Kazuaki Yazawa###

Thermoelectric properties of cement composite analogues from first principles calculations. Buildings are responsible for a considerable fraction of the energy wasted
globally every year, and as a result, excess carbon emissions. While heat is
lost directly in colder months and climates, resulting in increased heating
loads, in hot climates cooling and ventilation is required. One avenue towards
improving the energy efficiency of buildings is to integrate thermoelectric
devices and materials within the fabric of the building to exploit the
temperature gradient between the inside and outside to do useful work.
Cement-based materials are ubiquitous in modern buildings and present an
interesting opportunity to be functionalised. We present a systematic
investigation of the electronic transport coefficients relevant to the
thermoelectric materials of the calcium silicate hydrate (C-S-H) gel analogue,
tobermorite, using Density Functional Theory calculations with the Boltzmann
transport method. The calculated values of the Seebeck coefficient are within
the typical magnitude (200 - 600 $\mu V/K$) indicative of a good thermoelectric
material. The tobermorite models are predicted to be intrinsically $p$-type
thermoelectric material because of the presence of large concentration of the
Si-O tetrahedra sites. The calculated electronic $ZT$ for the tobermorite
models have their optimal values of 0.983 at (400 $\mathrm{K}$ and $10^{17}$
$\mathrm{cm^{-3}}$) for tobermorite 9 \r{A}, 0.985 at (400 $\mathrm{K}$ and
$10^{17}$ $\mathrm{cm^{-3}}$) for tobermorite 11 \r{A} and 1.20 at (225
$\mathrm{K}$ and $10^{19}$ $\mathrm{cm^{-3}}$) for tobermorite 14 \r{A},
respectively.

###NaInX2 (X = S, Se) layered materials for energy harvesting applications: First-principles insights into optoelectronic and thermoelectric properties|M. M. Hossain,M. A. Hossain,S. A. Moon,M. A. Ali,M. M. Uddin,S. H. Naqib,A. K. M. A. Islam M. Nagao S. Watauchi,I. Tanaka###

NaInX2 (X = S, Se) layered materials for energy harvesting applications: First-principles insights into optoelectronic and thermoelectric properties. In the present study, the structural, electronic, optical and thermoelectric
properties of two isostructural chalcogenide materials, NaInS2 and NaInSe2 with
hexagonal symmetry (R-3m) have been studied using the first principles method.
A very good agreement has been found between our results with the available
experimental and theoretical ones. The studied materials are semiconducting in
nature as confirmed from the electronic band structure and optical
properties.The strong hybridizations among s orbitals of Na, In and Se atoms
push the bottom of the conduction band downward resulting in a narrower band
gap of NaInSe2 compared to that of NaInS2 compound. Different optical
(dielectric function, photoconductivity, absorption coefficient, reflectivity,
refractive index and loss function) and thermoelectric (Seebeck coefficient,
electrical conductivity, power factor and thermal conductivity) properties of
NaInX2 (X = S, Se) have been studied in detail for the first time. It is found
that all these properties are significantly anisotropic due to the strongly
layered structure of NaInX2 (X = S, Se). Strong optical absorption with sharp
peaks is found in the far visible to mid ultraviolet (UV) regions while the
reflectivity is low in the UV region for both the compounds. Such features
indicate feasibility of applications in optoelectronic sector.The calculated
thermoelectric power factors at 1000 K for NaInS2 and NaInSe2 along a-axis are
found to be 151.5 micro Watt /cmK2 and 154 micro Watt/cmK2, respectively and
the corresponding ZT values are ~0.70. The obtained thermal conductivity along
a-axis for both compounds is high (~22 W/mK).This suggests that the reduction
of such high thermal conductivity is important to achieve higher ZT values of
the NaInX2(X = S, Se) compounds.

###Chemical trends in the high thermoelectric performance of the pyrite-type dichalcogenides: ZnS2, CdS2 and CdSe2|Tiantian Jia,Jesús Carrete,Georg K. H. Madsen,Yongsheng Zhang,Suhuai Wei###

Chemical trends in the high thermoelectric performance of the pyrite-type dichalcogenides: ZnS2, CdS2 and CdSe2. The thermoelectric properties of the three pyrite-type IIB-VIA2
dichalcogenides (ZnS2, CdS2 and CdSe2) are systematically investigated and
compared with those of the prototype ZnSe2 in order to optimize their
thermoelectric properties. Using the phonon Boltzmann transport equation, we
find that they all have ultralow lattice thermal conductivities. By analyzing
their vibrational properties, these are attributed to soft phonon modes derived
from the loosely bound rattling-like metal atoms and to strong anharmonicities
caused by the vibrations of all atoms perpendicular to the strongly bound
nonmetallic dimers. Additionally, by correlating those properties along the
series, we elucidate a number of chemical trends. We find that heavier atom
masses, larger atomic displacement parameters and longer bond lengths between
metal and nonmetal atoms can be beneficial to the looser rattling of the metal
atoms and therefore lead to softer phonon modes, and that stronger nonmetallic
dimer bonds can boost the anharmonicities, both leading to lower thermal
conductivities. Furthermore, we find that all three compounds have complex
energy isosurfaces at valence and conduction band edges that simultaneously
allow for large density-of-states effective masses and small conductivity
effective masses for both p-type and n-type carriers. Consequently, the
calculated thermoelectric figures of merit (ZT), can reach large values both
for p-type and n-type doping. Our study illustrates the effects of
rattling-like metal atoms and localized nonmetallic dimers on the thermal
transport properties and the importance of different carrier effective masses
to electrical transport properties in these pyrite-type dichalcogenides, which
can be used to predict and optimize the thermoelectric properties of other
thermoelectric compounds in the future.

###Influence of thermal environment on optimal working conditions of thermoelectric generators|Y. Apertet,H. Ouerdane,C. Goupil,Ph. Lecoeur###

Influence of thermal environment on optimal working conditions of thermoelectric generators. Optimization analyses of thermoelectric generators operation is of importance
both for practical applications and theoretical considerations. Depending on
the desired goal, two different strategies are possible to achieve high
performance: through optimization one may seek either power output maximization
or conversion efficiency maximization. Recent literature reveals the persistent
flawed notion that these two optimal working conditions may be achieved
simultaneously. In this article, we lift all source of confusion by correctly
posing the problem and solving it. We assume and discuss two possibilities for
the environment of the generator to govern its operation: constant incoming
heat flux, and constant temperature difference between the heat reservoirs. We
demonstrate that, while power and efficiency are maximized simultaneously if
the first assumption is considered, this is not possible with the second
assumption. This latter corresponds to the seminal analyses of Ioffe who put
forth and stressed the importance of the thermoelectric figure of merit $ZT$.
We also provide a simple procedure to determine the different optimal design
parameters of a thermoelectric generator connected to heat reservoirs through
thermal contacts with a finite and fixed thermal conductance.

###Investigation of the Thermoelectric Properties of ZnV$_{2}$O$_{4}$ Compound in High Temperature Region|Saurabh Singh,R. K. Maurya,Sudhir K. Pandey###

Investigation of the Thermoelectric Properties of ZnV$_{2}$O$_{4}$ Compound in High Temperature Region. In the present work, we report the experimental thermopower ($\alpha$) data
for ZnV$_{2}$O$_{4}$ compound in the high temperature range 300-600 K. The
value of $\alpha$ is found to be $\sim$184 and $\sim$126 $\mu$V/K at $\sim$300
and $\sim$600 K, respectively. The temperature dependent behavior of $\alpha$
is almost linear in the measured temperature range. To understand the large and
positive $\alpha$ value observed in this compound, we have also investigated
the electronic and thermoelectric properties by combining the
\textit{ab-initio} electronic structures calculations with Boltzmann transport
theory. Within the local spin density approximation plus Hubbard U, the
anti-ferromagnetic ground state calculation gives an energy gap $\sim$0.33 eV
for U=3.7 eV, which is in accordance with the experimental results. The
effective mass for holes in the valance band is found nearly four times that of
electrons in conduction band. The large effective mass of holes are mainly
responsible for the observed positive and large $\alpha$ value in this
compound. There is reasonably good matching between calculated and experimental
$\alpha$ data in the temperature range 300-410 K. The power factor calculation
shows that thermoelectric properties in high temperature region can be enhanced
by tuning the sample synthesis conditions and suitable doping. The estimated
value of \textit{figure-of-merit}, ZT, at different absolute temperature
suggest that ZnV$_{2}$O$_{4}$ compound can be a good thermoelectric material in
high temperature range.

###Enhancement of the thermoelectric properties in doped FeSb$_2$ bulk crystals|Kefeng Wang,Rongwei Hu,John Warren,C. Petrovic###

Enhancement of the thermoelectric properties in doped FeSb$_2$ bulk crystals. Kondo insulator FeSb$_2$ with large Seebeck coefficient would have potential
in thermoelectric applications in cryogenic temperature range if it had not
been for large thermal conductivity $\kappa$. Here we studied the influence of
different chemical substitutions at Fe and Sb site on thermal conductivity and
thermoelectric effect in high quality single crystals. At $5\%$ of Te doping at
Sb site thermal conductivity is suppressed from $\sim 250$ W/Km in undoped
sample to about 8 W/Km. However, Cr and Co doping at Fe site suppresses thermal
conductivity more slowly than Te doping, and even at 20$\%$ Cr/Co doping the
thermal conductivity remains $\sim 30$ W/Km. The analysis of different
contributions to phonon scattering indicates that the giant suppression of
$\kappa$ with Te is due to the enhanced point defect scattering originating
from the strain field fluctuations. In contrast, Te-doping has small influence
on the correlation effects and then for small Te substitution the large
magnitude of the Seebeck coefficient is still preserved, leading to the
enhanced thermoelectric figure of merit ($ZT\sim 0.05$ at $\sim 100$ K) in
Fe(Sb$_{0.9}$Te$_{0.1}$)$_2$.

###High Seebeck coefficient and ultra-low lattice thermal conductivity in Cs2InAgCl6|Enamul Haque,M. Anwar Hossain###

High Seebeck coefficient and ultra-low lattice thermal conductivity in Cs2InAgCl6. The elastic, electronic and thermoelectric properties of indium-based
double-perovskite halide, Cs2InAgCl6 have been studied by first principles
study. The Cs2InAgCl6 is found to be elastically stable, ductile, anisotropic
and relatively low hard material. The calculated direct bandgap 3.67 eV by
TB-mBJ functional fairly agrees with the experimentally measured value 3.3 eV
but PBE functional underestimates the bandgap by 1.483 eV. The relaxation time
and lattice thermal conductivity have been calculated by using relaxation time
approximation (RTA) within the supercell approach. The lattice thermal
conductivity (\k{appa}l) is quite low (0.2 Wm-1K-1). The quite low phonon group
velocity in the large weighted phase space, and high anharmonicity (large
phonon scattering) are responsible for small \k{appa}l. The room temperature
Seebeck coefficient is 199 {\mu}VK-1. Such high Seebeck coefficient arises from
the combination of the flat conduction band and large bandgap. We obtain power
factors at 300K by using PBE and TB-mBJ potentials are ~29 and ~31 mWm-1K-2,
respectively and the corresponding thermoelectric figure of merit of Cs2BiAgCl6
are 0.71 and 0.72. However, the maximum ZT value obtained at 700K is ~0.74 by
TB-mBJ potential. The obtained results implies that Cs2InAgCl6 is a promising
material for thermoelectric device applications.

###Spin caloritronics with superconductors: Enhanced thermoelectric effects, generalized Onsager response-matrix, and thermal spin currents|Jacob Linder,Marianne Etzelmüller Bathen###

Spin caloritronics with superconductors: Enhanced thermoelectric effects, generalized Onsager response-matrix, and thermal spin currents. It has recently been proposed and experimentally demonstrated that it is
possible to generate large thermoelectric effects in ferromagnet/superconductor
structures due to a spin-dependent particle-hole asymmetry. Here, we
theoretically show that quasiparticle tunneling between two spin-split
superconductors enhances the thermoelectric response manyfold compared to when
only one such superconductor is used, generating Seebeck coefficients
($\mathcal{S} > 1$ mV/K) and figures of merit ($ZT \simeq 40$) far exceeding
the best bulk thermoelectric materials, and also becomes more resilient toward
inelastic scattering processes. We present a generalized Onsager
response-matrix which takes into account spin-dependent voltage and temperature
gradients. Moreover, we show that thermally induced spin currents created in
such junctions, even in the absence of a polarized tunneling barrier, also
become largest in the case where a spin-dependent particle-hole asymmetry
exists on both sides of the barrier. We determine how these thermal spin
currents can be tuned both in magnitude and sign by several parameters,
including the external field, temperature, and the superconducting
phase-difference.

###Interference enhanced thermoelectricity in quinoid type structures|M. Strange,J. S. Seldenthuis,C. J. O. Verzijl,J. M. Thijssen,G. C. Solomon###

Interference enhanced thermoelectricity in quinoid type structures. Quantum interference (QI) effects in molecular junctions may be used to
obtain large thermoelectric responses. We study the electrical conductance G
and the thermoelec- tric response of a series of molecules featuring a quinoid
core using density functional theory (DFT), as well as a semi-empirical
interacting model Hamiltonian describing the {\pi}-system of the molecule which
we treat in the GW approximation. Molecules with a quinoid type structure are
shown to have two distinct destructive QI features close to the frontier
orbital energies. These manifest themselves as two dips in the transmission,
that remain separated, even when either electron donating or withdraw- ing side
groups are added. We find that the position of the dips in the transmission and
the frontier molecular levels can be chemically controlled by varying the
electron donating or withdrawing character of the side groups as well as the
conjugation length inside the molecule. This feature results in a very high
thermoelectric power factor S^2G and figure of merit ZT, where S is the Seebeck
coefficient, making quinoid type molecules potential candidates for efficient
thermoelectric devices.

###Significant ZT Enhancement in p-type Ti(Co,Fe)Sb-InSb Nanocomposites via a Synergistic High Mobility Electron Injection Energy filtering and Boundary Scattering Approach|Wenjie Xie,Yonggao Yana,Song Zhuc,Menghan Zhouc,Sascha Populohb,Krzysztof Gałązkab,S. Joseph Poon,Anke Weidenkaff,Jian He,Xinfeng Tanga,Terry M. Tritt###

Significant ZT Enhancement in p-type Ti(Co,Fe)Sb-InSb Nanocomposites via a Synergistic High Mobility Electron Injection Energy filtering and Boundary Scattering Approach. It has been demonstrated that InSb nanoinclusions, which are formed in situ,
can simultaneously improve all three individual thermoelectric properties of
the n-type half Heusler compound (Ti,Zr,Hf)(Co,Ni)Sb [Xie et al., Acta Mater.
58, 4795 (2010)]. In the work presented herein, we have adopted the same
approach to the p-type half Heusler compound Ti(Co,Fe)Sb. The results of
resistivity, Seebeck coefficient, thermal conductivity, and Hall coefficient
measurements indicate that the combined high mobility electron injection, low
energy electron filtering, and boundary scattering, again, lead to a
simultaneous improvement of all three individual thermoelectric properties:
enhanced Seebeck coefficient and electrical conductivity as well as reduced
lattice thermal conductivity. A figure of merit of ZT=0.33 was attained at 900
K for the sample containing 1 atomic percent InSb nanoinclusions, a 450 percent
improvement over the nanoinclusion-free sample. This represents a rare case
that the same nanostructuring approach successfully works for both p-type and
n-type thermoelectric materials of the same class, hence pointing to a
promising materials design route for higher performance half-Heusler materials
in the future and hopefully will realize similar improvement in TE devices
based on such half Heusler alloys.

###Effects of Confinement and Orientation on the Thermoelectric Power Factor of Silicon Nanowires|Neophytos Neophytou,Hans Kosina###

Effects of Confinement and Orientation on the Thermoelectric Power Factor of Silicon Nanowires. It is suggested that low dimensionality can improve the thermoelectric (TE)
power factor of a device, offering an enhancement of the ZT figure of merit. In
this work the atomistic sp3d5s*-spin-orbit-coupled tight-binding model and the
linearized Boltzmann transport theory is applied to calculate the room
temperature electrical conductivity, Seebeck coefficient, and power factor of
narrow 1D silicon nanowires (NWs). We present a comprehensive analysis of the
thermoelectric coefficients of n-type and p-type NWs of diameters from 12nm
down to 3nm, in [100], [110], and [111] transport orientations at different
carrier concentrations. We find that the length scale at which the influence of
confinement on the power factor can be observed is at diameters below 7nm. We
show that contrary to the current view, the effect of confinement and geometry
on the power factor originates mostly from changes in the conductivity which is
strongly affected, rather than the Seebeck coefficient. In general, enhanced
scattering at these diameter scales strongly degrades the conductivity and
power factor of the device. We identify cases, however, for which confinement
largely improves the channel's conductivity, resulting in ~2-3X power factor
improvements. Our results may provide guidance in the design of efficient low
dimensional thermoelectric devices.

###Tuning Electrical and Thermal Transport in AlGaN/GaN Heterostructures via Buffer Layer Engineering|Ananth Saran Yalamarthy,Hongyun So,Miguel Muñoz Rojo,Ateeq J. Suria,Xiaoqing Xu,Eric Pop,Debbie G. Senesky###

Tuning Electrical and Thermal Transport in AlGaN/GaN Heterostructures via Buffer Layer Engineering. Over the last decade, progress in wide bandgap, III-V materials systems based
on gallium nitride (GaN) has been a major driver in the realization of high
power and high frequency electronic devices. Since the highly conductive,
two-dimensional electron gas (2DEG) at the AlGaN/GaN interface is based on
built-in polarization fields (not doping) and is confined to very small
thicknesses, its charge carriers exhibit much higher mobilities in comparison
to their doped counterparts. In this study, we show that this heterostructured
material also offers the unique ability to manipulate electrical transport
separately from thermal transport through the examination of fully-suspended
AlGaN/GaN diaphragms of varied GaN buffer layer thicknesses. Notably, we show
that ~$100$ nm thin GaN layers can considerably impede heat flow without
electrical transport degradation, and that a significant improvement (~$4$x) in
the thermoelectric figure of merit ($\it zT$) over externally doped GaN is
observed in 2DEG based heterostructures. We also observe state-of-the art
thermoelectric power factors ($4-7\times$ $10^{-3}$$\,Wm^{-1}K^{-2}$) at room
temperature) in the 2DEG of this material system. This remarkable tuning
behavior and thermoelectric enhancement, elucidated here for the first time in
a polarization-based heterostructure, is achieved since the electrons are at
the heterostructured interface, while the phonons are within the material
system. These results highlight the potential for using the 2DEG in III-V
materials for on-chip thermal sensing and energy harvesting.

###Quantum transport simulations for the thermoelectric power factor in two dimensional nanocomposites|Samuel Foster,Mischa Thesberg,Neophytos Neophytou###

Quantum transport simulations for the thermoelectric power factor in two dimensional nanocomposites. Some of the most promising candidates for next generation thermoelectrics are
nanocomposites due to their low thermal conductivities that result from phonon
scattering on the boundaries of the various material phases. However, in order
to maximize the figure of merit ZT, it is important to understand the impact of
such features on the thermoelectric power factor. In this work we consider the
effect that nanoinclusions and voids have on the electronic and thermoelectric
coefficients of two dimensional geometries using the fully quantum mechanical
Non Equilibrium Greens Function method. This method combines in a unified
approach the details of geometry, electron phonon interactions, quantisation,
tunnelling, and the ballistic to diffusive nature of transport. We show that as
long as the barrier height is low nanoinclusions can have a positive impact on
the Seebeck coefficient and the power factor is not severely impacted by a
reduction in conductance. The power factor is also shown to be approximately
independent of nanoinclusion and void density in the ballistic case. On the
other hand, in the presence of phonon scattering voids degrade the power factor
and their influence increases with density.

###An ab-initio study on physical properties of Pd2+ incorporated double perovskites CaPd3B4O12 (B = Ti, V)|Mirza H. K. Rubel,Khandaker Monower Hossain,Anjuman Ara Khatun,M. Anwar Hossain,M. M. Rahaman,M. Mozahar Ali,M. M. Hossain,J. Hossain,Md. Rasadujjaman,S. Kojima,N. Kumada###

An ab-initio study on physical properties of Pd2+ incorporated double perovskites CaPd3B4O12 (B = Ti, V). Numerous physical properties of CaPd3Ti4O12 (CPTO) and CaPd3V4O12 (CPVO)
double perovskites have been explored based on density functional theory (DFT).
The calculated structural parameters fairly agree with the experimental data to
confirm their stability. The mechanical stability of these two compounds was
clearly observed by the Born stability criteria. To rationalize the mechanical
behavior, we investigate elastic constants, bulk, shear and Young's modulus,
Pugh's ratio, Poisson's ratio and elastic anisotropy index. The ductility index
confirms that both materials are ductile in nature. The electronic band
structure of CPTO and CPVO reveals the direct band gap semiconducting in nature
and metallic characteristics, respectively. The calculated partial density of
states indicates the strong hybridization between Pd 4d and O 2p orbital
electrons for CPTO and Pd 4d and V 3d O 2p for CPVO. The study of electronic
charge density map confirms the coexistence of covalent, ionic and metallic
bonding for both compounds. Fermi surface calculation of CPVO ensures both
electron and hole like surfaces indicating the multiple band nature. In the
midst of optical properties, photoconductivity and absorption coefficient of
both compounds reveal well qualitative compliance with consequences of band
structure computations. Among the thermodynamic properties, the Debye
temperature has been calculated to correlate its topical features including
thermoelectric behavior. The studied thermoelectric transport properties of
CPTO yielded the Seebeck coefficient (186 microVK-1), power factor (11.9
microWcm-1K-2) and figure of merit (ZT) value of about 0.8 at 800 K indicate
that this material could be a promising candidate for thermoelectric device
application.

###Optical excitations and thermoelectric properties of 2D holey graphene|Deobrat Singh,Vivekanand Shukla,Rajeev Ahuja###

Optical excitations and thermoelectric properties of 2D holey graphene. Recently, holey graphene (HG) has successfully synthesized at atomic
precision of hole size and shape. This shows interesting physical and chemical
properties for energy and environmental applications. Shaping of the pores also
transforms semimetallic graphene to semiconductor holey graphene, which opens
new door for its use in electronic applications. We systematically investigated
the structural, electronic, optical and thermoelectric properties of HG
structure using first-principles calculations. HG was found to have a direct
band gap with 0.65 eV (PBE functional), 0.95 eV (HSE06 functional) and HSE06
functional is in good agreement with experimental results. For the optical
properties, we use single-shot G0W0 calculations by solving the Bethe-Salpeter
equation to determining the intralayer excitonic effects. From the absorption
spectrum, we obtained the optical gap of 1.28 eV and a week excitonic binding
energy of 80 meV. We have found the large values of thermopower of 1662.59
$\mu$V/K and better electronic figure of merit, ZT$_{e}$ as 1.13 from the
investigated thermoelectric properties. Our investigations exhibit strong and
broad optical absorption in the visible light region, which makes HG monolayer
a promising candidate for optoelectronic and thermoelectric applications.

###Excellent Thermoelectric and Piezoelectric Properties of Differently Stacked Layers of Two-Dimensional Transition Metal Dinitride HfN2|Atanu Betal,Jayanta Bera,Satyajit Sahu###

Excellent Thermoelectric and Piezoelectric Properties of Differently Stacked Layers of Two-Dimensional Transition Metal Dinitride HfN2. Two-dimensional (2D) transition metal dinitride (HfN2) has been studied for
their optoelectronic, piezoelectric, and thermoelectric properties. Both
monolayer and bilayer of HfN2 were studied using density functional theory
(DFT) and Boltzmann transport equation (BTE). The bilayer of HfN2 with
different stacking layers (AA and AB) showed different electronic properties.
The optical property of the material suggests that it is a very good absorber
in the ultraviolet (UV) region thus, can be used as a UV-photodetector and as
an absorber layer in photovoltaic devices. The piezoelectric properties of the
material also showed promising behavior as the piezoelectric stress and strain
tensors have highest value of 8.97*(10)^(-10) C/m and 12.59 pm/V respectively
for the bilayer. The piezoelectric tensors have highest value for AB stacked
bilayer. The ZT value of 0.8 at 900 K is also highest for bilayer AB stacked
HfN2. These high values of piezoelectric and thermoelectric parameters of the
material suggest that the material would be an excellent choice as
thermoelectric energy harvesting devices as well as mechanical stress sensor or
actuator.

###Large Nernst effect and field enhanced transversal ZT in ZrTe5|Wang Peipei,Cho Chang-woo,Tang Fangdong,Wang Peng,Zhang Wenjie,He Mingquan,Gu Genda,Wu Xiaosong,Shao Yonghong,Zhang Liyuan###

Large Nernst effect and field enhanced transversal ZT in ZrTe5. Thermoelectric materials can recover electrical energy from waste heat and
vice versa, which are of great significance in green energy harvesting and
solid state refrigerator. The thermoelectric figure of merit (zT) quantifies
the energy conversion efficiency, and a large Seebeck or Nernst effect is
crucial for the development of thermoelectric devices. Here we present a
significantly large Nernst thermopower in topological semimetal ZrTe5, which is
attributed to both strong Berry curvature and bipolar transport. The largest
in-plane S_xy (when B//b) approaches 1900 {\mu}V/K at T=100K and B=13T, and the
out-of-plane S_xz (when B//c) reaches 5000 {\mu}V/K. As a critical part of z_N
T, the linearly increased in-plane S_xy and resistivity \r{ho}_yy regard to B
induces an almost linear increasing transversal z_N T without saturate under
high fields. The maximum z_N T of 0.12 was obtained at B=13 T and T= 120K,
which significantly surmounts its longitudinal counterpart under the same
condition.

###Thermoelectric properties of Sm-doped BiCuSeO oxyselenides fabricated by two-step reactive sintering|Andrei Novitskii,Illia Serhiienko,Sergey Novikov,Kirill Kuskov,Daria Pankratova,Tatyana Sviridova,Andrei Voronin,Aleksei Bogach,Elena Skryleva,Yuriy Parkhomenko,Alexander Burkov,Takao Mori,Vladimir Khovaylo###

Thermoelectric properties of Sm-doped BiCuSeO oxyselenides fabricated by two-step reactive sintering. Among layered oxygen-containing compounds, BiCuSeO is one of the most
promising candidates for thermoelectric applications due to its intrinsically
low thermal conductivity and good thermal stability. However, the rather poor
electrical conductivity of pristine BiCuSeO hinders its potential. Further
enhancement of the thermoelectric performance by single doping at Bi site is
limited mainly due to dramatic decrease of carrier mobility. Thus, new
strategies, such as dual doping or doping with variable-valence elements seem
to be promising. Along with that, the development of a fast and scalable
synthesis route is essential for the industrial-scale fabrication of
thermoelectric materials. Hence, in this paper, Bi$_{1-x}$Sm$_{x}$CuSeO samples
(0 $\leq$ $x$ $\leq$ 0.08) have been synthesized with a simple and scalable
reactive sintering process. For comparison, Bi$_{1-x}$Sm$_{x}$CuSeO
oxyselenides were also obtained by the conventional solid-state route. Our
results highlight that, Sm for Bi substitution increases the electrical
conductivity by 1.5 - 2 times and decreases the Seebeck coefficient by ~1.4
times at 873 K for both series. Overall, considering the increase of lattice
thermal conductivity upon doping and not optimized power factor, the figure of
merit $zT$ is reducing upon doping.

###Data-driven reconstruction of spectral conductivity and chemical potential from thermoelectric transport data|Tomoki Hirosawa,Frank Schäfer,Hideaki Maebashi,Hiroyasu Matsuura,Masao Ogata###

Data-driven reconstruction of spectral conductivity and chemical potential from thermoelectric transport data. The spectral conductivity, i.e., the electrical conductivity as a function of
the Fermi energy, is a cornerstone in determining the thermoelectric transport
properties of electrons. However, the spectral conductivity depends on
sample-specific properties such as carrier concentrations, vacancies, charge
impurities, chemical compositions, and material microstructures, making it
difficult to relate the experimental result with the theoretical prediction
directly. Here, we propose a data-driven approach based on machine learning to
reconstruct the spectral conductivity and chemical potential from the
thermoelectric transport data. Using this machine learning method, we first
demonstrate that the spectral conductivity and temperature-dependent chemical
potentials can be recovered within a simple toy model. In a second step, we
apply our method to experimental data in doped one-dimensional telluride
Ta$_4$SiTe$_4$~[T. Inohara, \textit{et al.}, Appl. Phys. Lett. \textbf{110},
183901 (2017)] to reconstruct the spectral conductivity and chemical potential
for each sample. Furthermore, the thermal conductivity of electrons and the
maximal figure of merit $ZT$ are estimated from the reconstructed spectral
conductivity, which provides accurate estimates beyond the Wiedemann-Franz law.
Our study clarifies the connection between the thermoelectric transport
properties and the low-energy electronic states of real materials, and
establishes a promising route to incorporate experimental data into traditional
theory-driven workflows.

###Thermoelectric effects in tunneling of spin-polarized electrons in a molecular transistor|A. D. Shkop###

Thermoelectric effects in tunneling of spin-polarized electrons in a molecular transistor. Thermal transmission in a molecular transistor with fully spin-polarized
electrodes subjected to a temperature gradient is considered. The problem has
been solved by using density matrix method in perturbation approach over small
tunneling width. It has been found that due to the vibronic effects spintronic
molecular transistor is characterized by negative differential
thermoconductance. It has been demonstrated that in the dependence of
thermopower $S$ on the detuning energy there is an increased number of points
of change of the sign and magnitude of $S$ comparing with that of conventional
molecular transistor. Optimal parameters, that provide the highest
thermoelectric power at maximum efficiency $P_{me}$ for spintronic molecular
transistor, have been found. The dependences of the figure of merit $ZT$ and
$P_{me}$ on temperature and an external magnetic field have been calculated and
the influence of Coulomb interaction on the thermolelectric properties has been
studied. It has been revealed, that for non-zero Coulomb interaction more handy
regime of thermoelectric device develops, characterized by continuous region of
external magnetic fields, which provide high values of thermoelectric power.

###Enhanced Thermoelectric Performance of Nanostructured Nickel Doped Ag2Te|Vikash Sharma,Divya Sharma,Ranu Bhatt,Pankaj Patro,Gunadhor Singh Okram###

Enhanced Thermoelectric Performance of Nanostructured Nickel Doped Ag2Te. We report on the thermoelectric properties of nickel doped Ag2-xNixTe (x = 0,
0.015, 0.025 & 0.055, 0.115, 0.155) nanostructures in the temperature (T) range
of 5 K to 575 K. The electrical resistivity of Ag2Te nanostructure shows
metallic behaviour in 5 K to 300 K initially that evolves into two metal to
insulator transitions (MITs) at low and mid-temperature regimes with increasing
x due to Mott-variable range hopping (VRH) and Arrhenius transports,
respectively. Their Seebeck coefficient varies nearly in a linear fashion in
this temperature range, showing metallic or doped-degenerate semiconducting
behaviour. Notably, this behaviour of the Seebeck coefficient is in contrast to
Mott VRH conduction as observed in resistivity. The steady increase in
resistivity and S with the sharp decrease in thermal conductivity between 410 K
to 425 K associated with the structural phase transition accomplishes a maximum
thermoelectric figure of merit (ZT) of 0.86 near 480 K in x = 0.155. This is
about 83 % more compared to that of bulk Ag2Te, and shows a significant
improvement over the best value reported for Ag2Te nanostructures thus far.
This study, therefore, shows that simultaneous nanocomposite formation, doping
and nanostructuring could be an effective strategy for tuning the electron and
phonon transports to improve the thermoelectric properties of a material.

###Double Half-Heusler Alloys X$_2$Ni$_2$InSb (X= Zr/Hf) with promising Thermoelectric Performance: Role of varying structural phases|Bhawna Sahni,Aftab Alam###

Double Half-Heusler Alloys X$_2$Ni$_2$InSb (X= Zr/Hf) with promising Thermoelectric Performance: Role of varying structural phases. Double half-heusler alloys are the new class of compounds which can be seen
as transmuted version of two single half-heusler with higher flexibility of
tuning their properties. Here, we report a detailed study of thermoelectric
(TE) properties of two double half-heusler (HH) alloys X$_2$Ni$_2$InSb
(X=Hf/Zr), using first-principles calculation. These alloys exhibit a rich
phase diagram with the possibility of tetragonal, cubic and solid solution
phase at different temperature range. As such, a comparative study of TE
properties of all these phases is performed. The ordered phases show quite
favorable electronic transport as compared to the disordered ones in both
compounds. Lattice thermal conductivity of double HH alloys is lower than their
ternary counter-part, making them most promising for TE application. Simulated
band gap, obtained using hybrid functional, of ordered phases of
Hf$_2$Ni$_2$InSb and Zr$_2$Ni$_2$InSb lie in the range 0.24-0.4 eV and
0.17-0.59 eV respectively, while for disordered phase, it is 0.05- 0.06 eV.
Hf$_2$Ni$_2$InSb shows a reasonably high ZT value of $\sim$ 2.19, while
Zr$_2$Ni$_2$InSb yields 2.46 at high temperature for n-type conduction in
tetragonal phase. The ZT value for p-type conduction is also quite promising
($\sim$ 1.35 and $\sim$ 2.19 for Hf- and Zr-based compounds). In both the
compounds, electronic transport (Seebeck and electrical conductivity) plays the
dominant role for the high ZT-value. Keeping in mind the promising TE
performance, we propose immediate attention from experimentalists to synthesize
and cross validate our findings for these new candidate materials.

###Distinctive Thermoelectric Properties of Supersaturated Si-Ge-P Compounds: Achieving Figure of Merit ZT > 3.6|Swapnil Ghodke,Omprakash Muthusamy,Kevin Delime Codrin,Seongho Choi,Saurabh Singh,Dogyun Byeon,Masahiro Adachi,Makoto Kiyama,Takashi Matsuura,Yoshiyuki Yamamoto,Masaharu Matsunami,Tsunehiro Takeuchi###

Distinctive Thermoelectric Properties of Supersaturated Si-Ge-P Compounds: Achieving Figure of Merit ZT > 3.6. The efficiency of energy conversion in thermoelectric generators (TEGs) is
directly proportional to electrical conductivity and Seebeck coefficient while
inversely to thermal conductivity. The challenge is to optimize these
interdependent parameters simultaneously. In this work, the problem is
addressed with a novel approach of nanostructuring and constructive electronic
structure modification to achieve a very high value of dimensionless figure of
merit ZT greater than 3.6 at 1000 K with negative Seebeck coefficient.
Supersaturated solid-solutions of Si-Ge containing 1 atomic percent Fe and 10
atomic percent P are prepared by high-energy ball milling. The bulk samples
consisting of ultra-fine nano-crystallites 9.7 nm are obtained by the
sophisticated low-temperature & high-pressure sintering process. Despite that
the electrical resistivity is slightly high due to the localization of
electrons is associated with the highly disordered structure and low electrical
density of states near the chemical potential, a very low thermal conductivity
\k{appa} less than 1 W m-1K-1 and very large magnitude of Seebeck coefficient
exceeding 470 uV K-1 are achieved in association with the nanostructuring and
the Fe 3d impurity states, respectively, to realize a very large magnitude of
ZT.

###Unexpectedly High Cross-plane Thermoelectric Performance in Layered Carbon Nitrides|Zhidong Ding,Meng An,Shenqiu Mo,Xiaoxiang Yu,Zelin Jin,Yuxuan Liao,Jingtao Lü,Kevian Esfarjani,Junichiro Shiomi,Nuo Yang###

Unexpectedly High Cross-plane Thermoelectric Performance in Layered Carbon Nitrides. Organic thermoelectric (TE) materials create a brand new perspective to
search for high-efficiency TE materials, due to their small thermal
conductivity. The overlap of pz orbitals, commonly existing in organic
{\pi}-stacking semiconductors, can potentially result in high electronic
mobility comparable to inorganic electronics. Here we propose a strategy to
utilize the overlap of pz orbitals to increase the TE efficiency of layered
polymeric carbon nitride (PCN). Through first-principles calculations and
classical molecular dynamics simulations, we find that A-A stacked PCN has
unexpectedly high cross-plane ZT up to 0.52 at 300 K, which can contribute to
n-type TE groups. The high ZT originates from its one-dimensional charge
transport and small thermal conductivity. The thermal contribution of the
overlap of pz orbitals is investigated, which noticeably enhances the thermal
transport when compared with the thermal conductivity without considering the
overlap effect. For a better understanding of its TE advantages, we find that
the low-dimensional charge transport results from strong pz-overlap
interactions and the in-plane electronic confinement, by comparing
{\pi}-stacking carbon nitride derivatives and graphite. This study can provide
a guidance to search for high cross-plane TE performance in layered materials.

###Electronic and Thermoelectric Properties of RuIn_{3-x}A_{x} (A = Sn, Zn)|Deepa Kasinathan,Maik Wagner,Klaus Koepernik,Raul Cardoso-Gil,Yu. Grin,Helge Rosner###

Electronic and Thermoelectric Properties of RuIn_{3-x}A_{x} (A = Sn, Zn). Recently, we reported [M. Wagner et al., J. Mater. Res. 26, 1886 (2011)]
transport measurements on the semiconducting intermetallic system RuIn3 and its
substitution derivatives RuIn_{3-x}A_{x} (A = Sn, Zn). Higher values of the
thermoelectric figure of merit (zT = 0.45) compared to the parent compound were
achieved by chemical substitution. Here, using density functional theory based
calculations, we report on the microscopic picture behind the measured
phenomenon. We show in detail that the electronic structure of the substitution
variants of the intermetallic system RuIn_{3-x}A_{x} (A = Sn, Zn) changes in a
rigid-band like fashion. This behavior makes possible the fine tuning of the
substitution concentration to take advantage of the sharp peak-like features in
the density of states of the semiconducting parent compound. Trends in the
transport properties calculated using the semi-classical Boltzmann transport
equations within the constant scattering time approximation are in good
agreement with the former experimental results for RuIn_{3-x}Sn_{x}. Based on
the calculated thermopower for the p-doped systems, we reinvestigated the
Zn-substituted derivative and obtained ZnO-free RuIn_{3-x}Zn_{x}. The new
experimental results are consistent with the calculated trend in thermopower
and yield large zT value of 0.8.

###The maximum theoretical performance of unconcentrated solar photovoltaic and thermoelectric generator systems|Rasmus Bjørk,Kaspar K. Nielsen###

The maximum theoretical performance of unconcentrated solar photovoltaic and thermoelectric generator systems. The maximum efficiency for photovoltaic (PV) and thermoelectric generator
(TEG) systems without concentration is investigated. Both a combined system
where the TEG is mounted directly on the back of the PV and a tandem system
where the incoming sunlight is split, and the short wavelength radiation is
sent to the PV and the long wavelength to the TEG, are considered. An
analytical model based on the Shockley-Queisser efficiency limit for PVs and
the TEG figure of merit parameter $zT$ is presented. It is shown that for
non-concentrated sunlight, even if the TEG operates at the Carnot efficiency
and the PV performance is assumed independent of temperature, the maximum
increase in efficiency is 4.5 percentage points (pp.) for the combined case and
1.8 pp. for the tandem case compared to a stand alone PV. For a more realistic
case with a temperature dependent PV and a realistic TEG, the gain in
performance is much lower. For the combined PV and TEG system it is shown that
a minimum $zT$ value is needed in order for the system to be more efficient
than a stand alone PV system.

###Thermoelectric and lattice dynamics properties of layered MX (M = Sn, Pb; X = S, Te) compounds|Abhiyan Pandit,Bothina Hamad###

Thermoelectric and lattice dynamics properties of layered MX (M = Sn, Pb; X = S, Te) compounds. Lead and tin chalcogenides have been studied widely due to their promising
thermoelectric (TE) properties. Further enhancement in their TE efficiency has
been reported upon the reduction of the dimension, which is an important
feature in modern device fabrications. Using density functional theory combined
with the Semi-classical Boltzmann transport theory, we studied the structural,
electronic and TE properties of two-dimensional (2D) MX (M = Sn, Pb; X = S, Te)
monolayers. Spin-orbit coupling was found to have significant effects on their
electronic structure, particularly for the heavy compounds. Structural
optimization followed by phonon transport studies prevailed that the
rectangular ({\gamma}-) phase is energetically the most favorable for SnS and
SnTe monolayers, whereas the square structure is found the most stable for PbS
and PbTe monolayers. Our results are in good agreement with previous studies.
These 2D materials exhibit high Seebeck coefficients and power factors along
with low lattice thermal conductivities, which are essential features of good
TE materials. The maximum figure of merits (ZT) of 1.04, 1.46, 1.51 and 1.94
are predicted for n-type SnS, SnTe, PBS and p-type PbTe monolayers respectively
at 700 K, which are higher than their bulk ZT values. Hence, these monolayers
are promising candidates for TE applications.

###Improved Thermoelectric Properties in (1-x)LaCoO3/(x)La0.7Sr0.3CoO3 Composite|Ashutosh Kumar,D. Sivaprahasam,Ajay D. Thakur###

Improved Thermoelectric Properties in (1-x)LaCoO3/(x)La0.7Sr0.3CoO3 Composite. A high Seebeck coefficient (S), large electrical conductivity ({\sigma}), and
reduced thermal conductivity ({\kappa}) are required to achieve a high
figure-of-merit (zT) in an ideal thermoelectric (TE) system, which is
challenging in a single system due to the interdependence of TE parameters.
Composite approach is promising to manipulate the TE parameters. In this study,
TE properties of (1-x)LaCoO3/(x)La0.7Sr0.3CoO3 (0.00 \leq x \leq 0.05)
composite is discussed. The structural analysis confirms individual phases in
the composite, which is further supported by electron microscopy analysis. The
x-ray photoelectron analysis indicates that oxygen vacancies (VO) are present
in the parent LaCoO3 system and increase with the addition of La0.7Sr0.3CoO3
(LSCO) in the composite. The increase in VO raises the degenerate states of
cobalt and hence improves S in the composites. Temperature variation in S and
{\sigma} are consistent with the spin-state transition and shows the
correlation between these two parameters. The reduction in {\kappa} and
{\sigma} with the addition of ball-milled La0.7Sr0.3CoO3 in the composite is
attributed to the enhanced phonon-phonon and charge carrier scattering,
respectively. A synergistic effect of enhanced S and reduced \kappa} result in
five times improvement in zT of the composite compared to the parent LaCoO3
system at 800 K. This approach also improves the operating temperature for
LaCoO3 based systems.

###Effect of Bi-substitution on Structural Stability and Improved Thermoelectric Performance of p-type Half-Heusler TaSbRu: A First-principles Study|Enamul Haque,Mostafizur Rahman,Parvin Sultana###

Effect of Bi-substitution on Structural Stability and Improved Thermoelectric Performance of p-type Half-Heusler TaSbRu: A First-principles Study. Recently, Fang et al. have predicted a high ZT of 1.54 in TaSbRu alloys at
1200 K from first-principles without considering spin-orbit interaction,
accurate electronic structure, details of phonon scattering, and
energy-dependent holes relaxation time. Here, we report the details of
structural stability and thermoelectric performance of Bi-Substituted p-type
TaSbRu from first-principles calculations considering theses important
parameters. This indirect bandgap semiconductor (Eg=0.8 eV by TB-mBJ+SOC) has
highly dispersive and degenerate valence bands, which lead to a maximum power
factor, 3.8 mWm-1K-2 at 300K. As Sb-5p has a small contribution to the bandgap
formation, the substitution of Bi on the Sb site does not cause significant
change to the electronic structure. Although the Seebeck coefficient increases
by Bi due to slight changes in the bandgap, electrical conductivity, and hence,
the power factor reduces to ~3 mW m-1K-2 at 300K (50% Bi). On the other side,
lattice thermal conductivity drops effectively to 5 from 20 W/m K as Bi
introduces a significant contribution in the acoustic phonon region and
intensify phonon scattering. Thus, ZT value is improved through
Bi-substitution, reaching 1.1 (50% Bi) at 1200 K from 0.45 (pure TaSbRu) only.
Therefore, the present study suggests how to improve the TE performance of
Sb-based half-Heusler compounds and TaSbRu (with 50% Bi) is a promising
material for high-temperature applications.

###Origins of minimized lattice thermal conductivity and enhanced thermoelectric performance in WS2/WSe2 lateral superlattice|Yonglan Hu,Tie Yang,Dengfeng Li,Guangqian Ding,Chaochao Dun,Dandan Wu,Xiaotian Wang###

Origins of minimized lattice thermal conductivity and enhanced thermoelectric performance in WS2/WSe2 lateral superlattice. We report a configuration strategy for improving the thermoelectric (TE)
performance of two-dimensional (2D) transition metal dichalcogenide (TMDC) WS2
based on the experimentally prepared WS2/WSe2 lateral superlattice (LS)
crystal. On the basis of density function theory combined with Boltzmann
transport equation, we show that the TE figure of merit zT of monolayer WS2 is
remarkably enhanced when forming into a WS2/WSe2 LS crystal. This is primarily
ascribed to the almost halved lattice thermal conductivity due to the enhanced
anharmonic processes. Electronic transport properties parallel (xx) and
perpendicular (yy) to the superlattice period are highly symmetric for both p-
and n-doped LS owing to the nearly isotropic lifetime of charger carriers. The
spin-orbital effect causes a significant split of conduction band and leads to
three-fold degenerate sub-bands and high density of states (DOS), which offers
opportunity to obtain the high n-type Seebeck coefficient (S). Interestingly,
the separated degenerate sub-bands and upper conduction band in monolayer WS2
form a remarkable stairlike DOS, yielding a higher S. The hole carriers with
much higher mobility than electrons reveal the high p-type power factor and the
potential to be good p-type TE materials with optimal zT exceeds 1 at 400K in
WS2/WSe2 LS.

###Enhanced Thermoelectric Performance of Polycrystalline Si0.8Ge0.2 Alloys through the Addition of Nanoscale Porosity|Hosseini,S. Aria,Romano,Giuseppe,Greaney,P. Alex###

Enhanced Thermoelectric Performance of Polycrystalline Si0.8Ge0.2 Alloys through the Addition of Nanoscale Porosity. Engineering materials to include nanoscale porosity or other nanoscale
structures has become a well-established strategy for enhancing the
thermoelectric performance of dielectrics. However, the approach is only
considered beneficial for materials where the intrinsic phonon mean-free path
is much longer than that of the charge carriers. As such, the approach would
not be expected to provide significant performance gains in polycrystalline
semiconducting alloys, such as SixGe1-x, where mass disorder and grains provide
strong phonon scattering. In this manuscript, we demonstrate that the addition
of nanoscale porosity to even ultrafine-grained Si0.8Ge0.2 may be worthwhile.
The semiclassical Boltzmann transport equation was used to model electrical and
phonon transport in polycrystalline Si0.8Ge0.2 containing prismatic pores
perpendicular to the transport current. The models are free of tuning
parameters and were validated against experimental data. The models reveal that
a combination of pores and grain boundaries suppresses phonon conductivity to a
magnitude comparable with the electronic thermal conductivity. In this regime,
ZT can be further enhanced by reducing carrier concentration to the electrical
and electronic thermal conductivity and simultaneously increasing thermopower.
Although increases in ZT are modest, the optimal carrier concentration is
significantly lowered, meaning semiconductors need not be so strongly
supersaturated with dopants.

###Bipolar conduction asymmetries lead to ultra-high thermoelectric power factor|Patrizio Graziosi,Zhen Li,Neophytos Neophytou###

Bipolar conduction asymmetries lead to ultra-high thermoelectric power factor. Low band gap thermoelectric materials suffer from bipolar effects at high
temperatures, with increased electronic thermal conductivity and reduced
Seebeck coefficient, leading to reduced power factor and low ZT figure of
merit. In this work we show that the presence of strong transport asymmetries
between the conduction and valence bands can allow high phonon-limited
electronic conductivity at finite Seebeck coefficient values, leading to
largely enhanced power factors. The power factors that can be achieved can be
significantly larger compared to their maximum unipolar counterparts, allowing
for doubling of the ZT figure of merit. We identify this behavior in low band
gap cases from the half-Heusler materials family. Using both, advanced
electronic Boltzmann transport calculations for realistic material
bandstructures, as well as model parabolic electronic bands, we elaborate on
the parameters that determine this effect. We then develop a series of
descriptors which can guide machine learning studies in identifying such
classes of materials with extraordinary power factors at nearly pristine
conditions. For this we test more than 3000 analytical bandstructures and their
features, and more than 120 possible descriptors, to identify the most
promising ones that contain: i) only band structure features for easy
identification from material databases, and ii) band structure and transport
parameters that provide much higher correlations, but for which parameter
availability can be somewhat scarce.

###Thermoelectric properties of Sb doped AlFe$_2$B$_2$|Duraisamy Sivaprahasam,Ashutosh Kumar,Babu Jayachandran,Raghavan Gopalan###

Thermoelectric properties of Sb doped AlFe$_2$B$_2$. In this work, thermoelectric properties of Al$_{1.2}$Fe$_2$B$_2$ compound
were investigated over a temperature range from 300\,K to 773\,K.
Al$_{1.2}$Fe$_2$B$_2$ compound was produced by vacuum arc melting of Al, Fe,
and B followed by annealing at 1323 K under argon atmosphere. The annealed
ingots were subsequently crushed into powder and hot pressed at 1273 K under
vacuum. The hot-pressed alloy predominantly contained Al$_{1.2}$Fe$_2$B$_2$
phase with a small fraction of FeB, which decreases further upon 0.1 \% Sb
doping in Al$_{1.2}$Fe$_2$B$_2$. The pristine Al$_{1.2}$Fe$_2$B$_2$ exhibits
n-type conductivity with a maximum figure of merit (zT) of 0.03 at 773\,K. The
Sb doping improves the Seebeck coefficient at high temperatures and also
reduces the phonon thermal conductivity across the temperature range studied.
The decrease in phonon thermal conductivity is attributed to the point-defect
phonon scattering due to mass fluctuation between the Fe and Sb atoms. The 0.1
at\% Sb doping at the Fe site results in improved zT of 0.056 at 773\,K in
spite of its limited dissolution in Al$_{1.2}$Fe$_2$B$_2$ and forms FeSb$_2$
secondary phase.

###Tuning lattice thermal conductance by porosity control in ultra-scaled Si and Ge nanowires|Abhijeet Paul,Gerhard Klimeck###

Tuning lattice thermal conductance by porosity control in ultra-scaled Si and Ge nanowires. Porous nanowires (NWs) with tunable thermal conductance are examined as a
candidate for thermoelectric (TE) devices with high efficiency (ZT). Thermal
conductance of porous Si and Ge NWs is calculated using the complete phonon
dispersion obtained from a modified valence force field (MVFF) model. The
presence of holes in the wires break the crystal symmetry which leads to the
reduction in ballistic thermal conductance ($\sigma_{l}$). $[100]$ Si and Ge
NWs show similar percentage reduction in $\sigma_{l}$ for the same amount of
porosity. A 4nm $\times$ 4nm Si (Ge) NW shows $\sim$ 30% (29%) reduction in
$\sigma_{l}$ for a hole of radius 0.8nm. The model predicts an anisotropic
reduction in $\sigma_{l}$ in SiNWs, with $[111]$ showing maximum reduction
followed by $[100]$ and $[110]$ for a similar hole radius. The reduction in
$\sigma_{l}$ is attributed to phonon localization and anisotropic mode
reduction.

###Phase Stability, Structures and Properties of the (Bi2)m(Bi2Te3)n Natural Superlattices|J. -W. G. Bos,F. Faucheux,R. A. Downie,A. Marcinkova###

Phase Stability, Structures and Properties of the (Bi2)m(Bi2Te3)n Natural Superlattices. The phase stability of the (Bi2)m(Bi2Te3)n natural superlattices has been
investigated through the low temperature solid state synthesis of a number of
new binary BixTe1-x compositions. Powder X-ray diffraction revealed that an
infinitely adaptive series forms for 0.44 < x < 0.70, while an unusual 2-phase
region with continuously changing compositions is observed for 0.41 < x < 0.43.
For x > 0.70, mixtures of elemental Bi and an almost constant composition
(Bi2)m(Bi2Te3)n phase are observed. Rietveld analysis of synchrotron X-ray
powder diffraction data collected on Bi2Te (m = 2, n = 1) revealed substantial
interchange of Bi and Te between the Bi2 and Bi2Te3 blocks, demonstrating that
the block compositions are variable. All investigated phase pure compositions
are degenerate semiconductors with low residual resistivity ratios and moderate
positive magnetoresistances (R/R0 = 1.05 in 9 T). The maximum Seebeck
coefficient is +80 muV K-1 for x = 0.63, leading to an estimated thermoelectric
figure of merit, zT = 0.2 at 250 K.

###Reduced thermal conductivity of TiNiSn/HfNiSn superlattices|Paulina Hołuj,Christoph Euler,Benjamin Balke,Ute Kolb,Gregor Fiedler,Mathis M. Müller,Tino Jaeger,Peter Kratzer,Gerhard Jakob###

Reduced thermal conductivity of TiNiSn/HfNiSn superlattices. Diminution of the thermal conductivity is a crucial aspect in thermoelectric
research. We report a systematic and significant reduction of the cross-plane
thermal conductivity in a model system consisting of DC sputtered TiNiSn and
HfNiSn half-Heusler superlattices. The reduction of $\kappa$ is measured by the
3$\omega$ method and originates from phonon scattering at the internal
interfaces. Heat transport in the superlattices is calculated based on
Boltzmann transport theory, including a diffusive mismatch model for the
phonons at the internal interfaces. Down to superlattice periodicity of 3 nm
the phonon spectrum mismatch between the superlattice components quantitatively
explains the reduction of $\kappa$. For very thin individual layers the
interface model breaks down and the artificial crystal shows an enhanced
$\kappa$. We also present an enhanced ZT value for all investigated
superlattices compared to the single TiNiSn and HfNiSn films.

###First-principles study of the thermoelectric properties of quaternary tetradymite BiSbSeTe2|Z. Z. Zhou,H. J. Liu,D. D. Fan,B. Y. Zhao,C. Y. Sheng,G. H. Cao,S. Huang###

First-principles study of the thermoelectric properties of quaternary tetradymite BiSbSeTe2. The electronic and phonon transport properties of quaternary tetradymite
BiSbSeTe2 are investigated using first-principles approach and Boltzmann
transport theory. Unlike the binary counterpart Bi2Te3, we obtain a pair of
Rashba splitting bands induced by the absence of inversion center. Such unique
characteristic could lead to a large Seebeck coefficient even at relatively
higher carrier concentration. Besides, we find an ultralow lattice thermal
conductivity of BiSbSeTe2, especially along the interlayer direction, which can
be traced to the extremely small phonon relaxation time mainly induced by the
mixed covalent bonds. As a consequence, a considerably large ZT value of ~2.0
can be obtained at 500 K, indicating that the unique lattice structure of
BiSbSeTe2 caused by isoelectronic substitution could be an advantage to
achieving high thermoelectric performance.

###Thermal conductivity reduction by acoustic Mie resonance in nanoparticles|Brian Slovick,Srini Krishnamurthy###

Thermal conductivity reduction by acoustic Mie resonance in nanoparticles. We evaluate the impact of acoustic Mie resonance in nanoparticles on the
thermal conductivity of semiconductor and polymer composites. By appropriately
choosing the bulk modulus and density, and selecting the size of the
nanoparticle to align the Mie resonances with the dominant portion of the
thermal conductivity spectrum, we show that large reductions in thermal
conductivity are achievable with dilute concentrations of nanoparticles. In
semiconductor alloys, where the spectral thermal conductivity is known, our
model can explain the steep reductions in thermal conductivity observed
previously. However, the results of our effort to evaluate acoustic Mie
resonance in polymer composites are inconclusive due to uncertainties in the
spectral thermal conductivity. Acoustic Mie resonances can be useful for
maximizing ZT for thermoelectric applications, since a dilute loading of
nanoparticles can reduce thermal conductivity with minimal impact on electrical
conductivity.

###High-throughput exploration of alloying as design strategy for thermoelectrics|Sandip Bhattacharya,Georg K. H. Madsen###

High-throughput exploration of alloying as design strategy for thermoelectrics. We explore a material design strategy to optimize the thermoelectric power
factor. The approach is based on screening the band structure changes upon a
controlled volume change. The methodology is applied to the binary silicides
and germanides. We first confirm the effect in antifluorite Mg2Si and Mg2Ge
where an increased power factor by alloying with Mg2Sn is experimentally
established. Within a high-throughput formalism we identify six previously
unreported binaries that exhibit an improvement in their transport properties
with volume. Among these, hexagonal MoSi2 and orthorhombic Ca2Si and Ca2Ge have
the highest increment in zT with volume. We then perform super-cell
calculations on special quasi-random structures to investigate the possibility
of obtaining thermodynamically stable alloy systems which would produce the
necessary volume changes. We find that for Ca2Si and Ca2Ge the solid solutions
with the isostructural Ca2Sn readily forms even at low temperatures.

###Basic concepts in single-molecule electronics|C J Lambert###

Basic concepts in single-molecule electronics. This tutorial outlines the basic theoretical concepts and tools which
underpin the fundamentals of phase-coherent electron transport through single
molecules. The key quantity of interest is the transmission coefficient T(E),
which yields the electrical conductance, current-voltage relations, the
thermopower S and the thermoelectric figure of merit ZT of single-molecule
devices. Since T(E) is strongly affected by quantum interference (QI), three
manifestations of QI in single-molecules are discussed, namely Mach-Zehnder
interferometry, Breit-Wigner resonances and Fano resonances. A simple MATLAB
code is provided, which allows the novice reader to explore QI in
multi-branched structures described by a tight-binding (Huckel) Hamiltonian.
More generally, the strengths and limitations of materials-specific transport
modelling based on density functional theory are discussed.

###Spin-dependent Seebeck effect and huge growth of thermoelectric parameters at band edges in H- and F-doped graphene, free-standing and deposited on 4H-SiC(0001) C-face|Malgorzata Wierzbowska,Adam Dominiak###

Spin-dependent Seebeck effect and huge growth of thermoelectric parameters at band edges in H- and F-doped graphene, free-standing and deposited on 4H-SiC(0001) C-face. Graphene halfly doped with H or F possesses local magnetization at the
undoped C sites. Thus the Seebeck coefficient is different for each spin
channel and its sign also changes depending on the spin polarization.
Deposition of doped graphene on the C-face 4H-SiC(0001) with two buffer layers
substantially varies the electronic and thermoelectric properties. These
properties are efficiently calculated from the semiclassical Boltzmann
equations, using the maximally-localized Wannier-functions interpolation of the
band structures obtained with the density-functional theory. Our results
indicate large growth of the thermopower and the ZT efficiency at the band
edges. We show in the model discussion that this phenomenon is more general and
applies also to other systems than graphene. It gives prospect for developing
new spintronic devices working in the band-edge regime.

###Ultra-low acoustic-phonon-limited mobility and giant phonon-drag thermopower in MgZnO/ZnO heterostructures|Margarita Tsaousidou###

Ultra-low acoustic-phonon-limited mobility and giant phonon-drag thermopower in MgZnO/ZnO heterostructures. We present numerical simulations of the acoustic-phonon-limited mobility,
$\mu_{ac}$, and phonon-drag thermopower, $S^{g}$, in two-dimensional electron
gases confined in MgZnO/ZnO heterostructures. The calculations are based on the
Boltzmann equation and are made for temperatures in the range 0.3-20 K and
sheet densities $0.5$-$30\times 10^{15}$ m$^{-2}$. The theoretical estimations
of $\mu_{ac}$ are in good agreement with the experiment without any adjustable
parameters. We find that the magnitude of $\mu_{ac}$ is dramatically decreased
in relation to GaAs based heterostructures. The phonon-drag thermopower,
$S^{g}$, which according to Herring's expression is inversely proportional to
$\mu_{ac}$ is severely increased exceeding 200 mV/K at $T=5$ K depending on
sheet density. The giant values of $S^{g}$ lead to a strong improvement of the
figure of merit $ZT$ at low temperatures. Our findings suggest that MgZnO/ZnO
heterostructures can be candidates for good thermoelectric materials at
cryogenic temperatures.

###MoS2 nanoribbons as promising thermoelectric materials|D. D. Fan,H. J. Liu,L. Cheng,P. H. Jiang,J. Shi,X. F. Tang###

MoS2 nanoribbons as promising thermoelectric materials. The thermoelectric properties of MoS2 armchair nanoribbons with different
width are studied by using first-principles calculations and Boltzmann
transport theory, where the relaxation time is predicted from deformation
potential theory. Due to the dangling bonds at the armchair edge, there is
obvious structure reconstruction of the nanoribbons which plays an important
role in governing the electronic and transport properties. The investigated
armchair nanoribbons are found to be semiconducting with indirect gaps, which
exhibit interesting width-dependent oscillation behavior. The smaller gap of
nanoribbon with width N = 4 leads to a much larger electrical conductivity at
300 K, which outweighs the relatively larger electronic thermal conductivity
when compared with those of N = 5, 6. As a results, the room temperature ZT
values can be optimized to 2.7 (p-type) and 2.0 (n-type), which significantly
exceed the performance of most laboratory results reported in the literature.

###Nanomagnet coupled to quantum spin Hall edge: An adiabatic quantum motor|Liliana Arrachea,Felix von Oppen###

Nanomagnet coupled to quantum spin Hall edge: An adiabatic quantum motor. The precessing magnetization of a magnetic islands coupled to a quantum spin
Hall edge pumps charge along the edge. Conversely, a bias voltage applied to
the edge makes the magnetization precess. We point out that this device
realizes an adiabatic quantum motor and discuss the efficiency of its operation
based on a scattering matrix approach akin to Landauer-B"uttiker theory.
Scattering theory provides a microscopic derivation of the
Landau-Lifshitz-Gilbert equation for the magnetization dynamics of the device,
including spin-transfer torque, Gilbert damping, and Langevin torque. We find
that the device can be viewed as a Thouless motor, attaining unit efficiency
when the chemical potential of the edge states falls into the
magnetization-induced gap. For more general parameters, we characterize the
device by means of a figure of merit analogous to the ZT value in
thermoelectrics.

###The role of ionized impurity scattering on the thermoelectric performances of rock salt AgPbmSnSe2+m|Lin Pan,Sunanda Mitra,Li-Dong Zhao,Yawei Shen,Yifeng Wang,Claudia Felser,David Berardan###

The role of ionized impurity scattering on the thermoelectric performances of rock salt AgPbmSnSe2+m. We report on the successful synthesis and on the properties of
polycrystalline AgPbmSnSe2+m (m = ++++, 100, 50, 25) samples with a rock salt
structure. Between 160 K and 400 K, the dominant scattering process of the
carriers in this system changes from acoustic phonon scattering in PbSe to
ionized impurity scattering in AgPbmSnSe2+m, which synergistically optimizes
electrical and thermal transport properties. Thanks to the faint amount of
AgSnSe2, the Seebeck coefficient is enhanced by boosting the scattering factor,
the electric conductivity is improved by the increase of the concentration of
holes coupled to a limited degradation of their mobility, and the total thermal
conductivity is reduced by suppressing bipolar thermal conductivity. Therefore,
ZT of AgPbmSnSe2+m (m = 50) reaches 1.3 at 889 K. The mechanism suggested in
this study opens new paths to improve the thermoelectric performances of other
families of materials.

###Ab initio thermal conductivity of thermoelectric Mg$_3$Sb$_2$: evidence for dominant extrinsic effects|Maria Barbara Maccioni,Roberta Farris,Vincenzo Fiorentini###

Ab initio thermal conductivity of thermoelectric Mg$_3$Sb$_2$: evidence for dominant extrinsic effects. The lattice thermal conductivity of the candidate thermoelectric material
Mg$_3$Sb$_2$ is studied from first principles, with the inclusion of
anharmonic, isotope, and boundary scattering processes, and via an accurate
solution of the Boltzmann equation. We find that the anomalously low observed
conductivity is due to grain-boundary scattering of phonons, whereas the purely
anharmonic conductivity is an order of magnitude larger. Mass disorder due to
alloying and off-stoichiometry is also found to contribute significantly to its
decrease. Combining ab initio values vs sample size with measured grain-size
distributions, we obtain an estimate of $\kappa$ vs T in nano-polycrystalline
material in good agreement with typical experiments, and compute the ZT figure
of merit in the various cases.

###High thermoelectric figure of merit and thermopower in layered perovskite oxides|Vincenzo Fiorentini,Roberta Farris,Edoardo Argiolas,Maria Barbara Maccioni###

High thermoelectric figure of merit and thermopower in layered perovskite oxides. We predict high thermoelectric efficiency in the layered perovskite
La$_2$Ti$_2$O$_7$, based on calculations (mostly ab-initio) of the electronic
structure, transport coefficients, and thermal conductivity in a wide
temperature range. The figure of merit $ZT$ computed with a
temperature-dependent relaxation time increases monotonically from just above 1
at room temperature to over 2.5 at 1200 K, at an optimal carrier density of
around 10$^{20}$ cm$^{-3}$. The Seebeck thermopower coefficient is between 200
and 300 $\mu$V/K at optimal doping, but can reach nearly 1 mV/K at low doping.
Much of the potential of this material is due to its lattice thermal
conductivity of order 1 W/(K m); using a model based on ab initio anharmonic
calculations, we interpret this low value as due to effective phonon
confinement within the layered-structure blocks.

###Graphene-like conjugated pi-bond system in Pb1-xSnxSe|G. J. Shu,S. C. Liou,S. Karna,R. Sankar,M. Hayashi,M. -W. Chu,F. C. Chou###

Graphene-like conjugated pi-bond system in Pb1-xSnxSe. Following the identification of the pi bond in graphene, in this work, a pi
bond constructed through side-to-side overlap of half-filled 6pz orbitals was
observed in a non-carbon crystal of Pb1-xSnxSe (x=0.34) (PSS), a prototype
topological crystalline insulator (TCI) and thermoelectric material with a high
figure-of-merit (ZT). PSS compounds with a rock-salt type cubic crystal
structure was found to consist of sigma bond connected covalent chains of
Pb(Sn)-Se with an additional pi bond that is shared as a conjugated system
among the four nearest neighbor Pb pairs in square symmetry within all (001)
monoatomic layers per cubic unit cell. The pi bond formed with half-filled 6pz
orbitals between Pb atoms is consistent with the calculated results from
quantum chemistry. The presence of pi bonds was identified and verified with
electron energy-loss spectroscopy (EELS) through plasmonic excitations and
electron density (ED) mapping via an inverse Fourier transform of X-ray
diffraction.

###Thermoelectric Transport in Graphene/$h$-BN/Graphene Heterostructures: A Computational Study|Ransell D'Souza,Sugata Mukherjee###

Thermoelectric Transport in Graphene/$h$-BN/Graphene Heterostructures: A Computational Study. We present first principles study of thermoelectric transport properties of
sandwiched heterostructure of Graphene (G)/hexagonal Boron Nitride (BN)/G,
based on Boltzmann transport theory for band electrons using the bandstructure
calculated from the Density Functional Theory (DFT) based plane-wave method.
Calculations were carried out for three, four and five BN layers sandwiched
between Graphene layers with three different arrangements to obtain the Seebeck
coefficient and Power factor in $T\sim 25-400$K range. Moreover, using
Molecular Dynamics (MD) simulations with very large simulation cell we obtained
the thermal conductance ($K$) of these heterostructures and obtained finally
the Figure-of-Merit ($ZT$). These results are in agreement with recently
reported experimental measurements.

###Earth-abundant and Non-toxic SiX (X=S, Se) Monolayers as Highly Efficient Thermoelectric Materials|Ji-Hui Yang,Qinghong Yuan,Huixiong Deng,Su-Huai Wei,Boris I. Yakobson###

Earth-abundant and Non-toxic SiX (X=S, Se) Monolayers as Highly Efficient Thermoelectric Materials. Current thermoelectric (TE) materials often have low performance or contain
less abundant and/or toxic elements, thus limiting their large-scale
applications. Therefore, new TE materials with high efficiency and low cost are
strongly desirable. Here we demonstrate that, SiS and SiSe monolayers made from
non-toxic and earth-abundant elements intrinsically have low thermal
conductivities arising from their low-frequency optical phonon branches with
large overlaps with acoustic phonon modes, which is similar to the
state-of-the-art experimentally demonstrated material SnSe with a layered
structure. Together with high thermal power factors due to their
two-dimensional nature, they show promising TE performances with large figure
of merit (ZT) values exceeding 1 or 2 over a wide range of temperatures. We
establish some basic understanding of identifying layered materials with low
thermal conductivities, which can guide and stimulate the search and study of
other layered materials for TE applications.

###Improving thermoelectric performance of TiNiSn by mixing MnNiSb in the half-Heusler structure|T. Berry,S. Ouardi,G. H. Fecher,B. Balke,G. Kreiner,G. Auffermann,W. Schnelle,C. Felser###

Improving thermoelectric performance of TiNiSn by mixing MnNiSb in the half-Heusler structure. The thermoelectric properties of n type semiconductor, TiNiSn is optimized by
partial substitution with metallic, MnNiSb in the half Heusler structure.
Herein, we study the transport properties and intrinsic phase separation in the
system. The Ti1-xMnxNiSn1-xSbx alloys were prepared by arc-melting and were
annealed at temperatures obtained from differential thermal analysis and
differential scanning calorimetry results. The phases were characterized using
powder X-ray diffraction patterns, energy dispersive X-ray spectroscopy, and
differential scanning calorimetry. After annealing the majority phase was
TiNiSn with some Ni rich sites and the minority phases was majorly Ti6Sn5, Sn,
and MnSn2. Ni rich sites were caused by Frenkel defects, this led to a
metal-like behavior of the semiconducting specimens at low temperature. For x
up to 0.05 the samples showed an activated conduction, whereas for x>0.05 they
showed metallic character. The figure of merit for x=0.05 was increased by 61%
(ZT=0.45) in comparison to the pure TiNiSn.

###Enhanced thermoelectricity at the ultra-thin film limit|Thao T. T. Nguyen,Linh T. Dang,Giang H. Bach,Tung H. Dang,Kien T. Nguyen,Hong T. Pham,Thuat T. Nguyen,Tuyen V. Nguyen,Toan T. Nguyen,Hung Q. Nguyen###

Enhanced thermoelectricity at the ultra-thin film limit. At the ultra-thin film limit, quantum confinement strongly improves
thermoelectric figure of merit in materials such as Sb$_2$Te$_3$ and
Bi$_2$Te$_3$. These high quality films have only been realized using well
controlled techniques such as molecular beam epitaxy. We report a two fold
increase in the Seebeck coefficient for both p-type Sb$_2$Te$_3$ and n-type
Bi$_2$Te$_3$ using thermal co-evaporation, an affordable approach. At the thick
film limit greater than 100 nm, their Seebeck coefficients are around 100 $\mu
V/K$, similar to results obtained in other work. When the films are thinner
than 50 nm, the Seebeck coefficient increases to about 500 $\mu V/K$. With a
total Seebeck coefficient $\sim$ 1 mV/K and an estimate ZT $\sim$ 2, this pair
of materials is the first step to a practical micro-cooler at room temperature.

###Impact of strain-induced electronic topological transition on the thermoelectric properties of PtCoO$_2$ and PdCoO$_2$|Markus Ernst Gruner,Ulrich Eckern,Rossitza Pentcheva###

Impact of strain-induced electronic topological transition on the thermoelectric properties of PtCoO$_2$ and PdCoO$_2$. By a combination of first-principles calculations and semi-classical
Boltzmann transport theory, we investigate the effect of epitaxial strain on
the electronic structure and transport properties of PtCoO$_2$ and PdCoO$_2$.
In contrast to the rather uniform elastic response of both systems, we predict
for PtCoO$_2$ a high sensitivity of the out-of-plane transport properties to
strain, which is not present in PdCoO$_2$. At ambient temperature, we identify
a considerable absolute change in the thermopower from $-107\,\mu$V/K at
$-5\,$\% compressive strain to $-303\,\mu$V/K at $+5\,$\% tensile strain. This
remarkable response is related to distinct changes of the Fermi surface, which
involve the crossing of two additional bands at a moderate compressive in-plane
strain. Combining our transport results with available experimental data on
electrical and lattice thermal conductivity we predict a thermoelectric figure
of merit of up to $ZT$$\,=\,$$0.25$ at $T$$\,=\,$$600\,$K for strained
PtCoO$_2$.

###Strong interlayer coupling in two-dimensional PbSe with high thermoelectric performance|Z. P. Yin,C. Y. Sheng,R. Hu,S. H. Han,D. D. Fan,G. H. Cao,H. J. Liu###

Strong interlayer coupling in two-dimensional PbSe with high thermoelectric performance. It was generally assumed that weak van der Waals interactions exist between
neighboring layers in the two-dimensional group-IV chalcogenides. Using PbSe as
a prototypal example, however, we find additional strong coupling between the
Pb-Pb layers, as evidenced by detailed analysis of the differential charge
density. The coupling resembles covalent-like bond and exhibits strong
harmonicity around the equilibrium distance, which can be fine tuned to
obviously reduce the phonon thermal conductivity but slightly change the
electronic transport of PbSe. As a consequence, a maximum ZT value of 2.5 can
be realized at 900 K for the p-type system. Our work offers an effective and
feasible design strategy to enhance the thermoelectric performance of similar
layered structures.

###Ultralow Thermal Conductivity and Large Figure of Merit in Low-Cost and Nontoxic Core-Shell Cu@Cu2O Nanocomposites|Vikash Sharma,Gunadhor Singh Okram,Divya Verma,Niranjan Prasad Lalla,Yung-Kang Kuo###

Ultralow Thermal Conductivity and Large Figure of Merit in Low-Cost and Nontoxic Core-Shell Cu@Cu2O Nanocomposites. Identification of novel materials with enhanced thermoelectric (TE)
performance is critical for advancing TE research. In this direction, this is
the first report on TE properties of low-cost, nontoxic, and abundant
core-shell Cu@Cu2O nanocomposites (NCs) synthesized using a facile and cheap
solution-phase method. They show ultralow thermal conductivity of nearly 10-3
of copper bulk value, large thermopower ~0.373 mVK-1, and consequently, a TE
figure of merit (ZT) of 0.16 at 320 K which is larger than those of many of the
potential TE materials such as PbTe, SnSe and SiGe, showing its potential for
TE applications. The ultralow thermal conductivity is mainly attributed to the
multiscale phonon scattering from intrinsic defects in Cu2O, grain boundaries
(GBs), lattice-mismatched interface as well as dissimilar vibrational
properties. The large thermopower is associated with sharp modulation in
carrier density of states (DOS) due to charge transfer between Cu and Cu2O
nanoparticles (NPs), and carrier energy filtering.

###Optimal Thermoelectric Power Factor of Narrow-Gap Semiconducting Carbon Nanotubes with Randomly Substituted Impurities|Manaho Matsubara,Kenji Sasaoka,Takahiro Yamamoto,Hidetoshi Fukuyama###

Optimal Thermoelectric Power Factor of Narrow-Gap Semiconducting Carbon Nanotubes with Randomly Substituted Impurities. We have theoretically investigated thermoelectric (TE) effects of narrow-gap
single-walled carbon nanotubes (SWCNTs) with randomly substituted nitrogen (N)
impurities, i.e., N-substituted (20,0) SWCNTs with a band gap of 0.497 eV. For
such a narrow-gap system, the thermal excitation from the valence band to the
conduction band contributes to its TE properties even at the room temperature.
In this study, the N-impurity bands are treated with both conduction and
valence bands taken into account self-consistently. We found the optimal N
concentration per unit cell, $c_{\rm opt}$, which gives the maximum power
factor ($PF$) for various temperatures, e.g., $PF=$0.30$\rm{W/K^2m}$ with
$c_{\rm opt}=3.1\times 10^{-5}$ at 300K. In addition, the electronic thermal
conductivity has been estimated, which turn out to be much smaller than the
phonon thermal conductivity, leading to the figure of merit as $ZT\sim 0.1$ for
N-substituted (20,0) SWCNTs with $c_{\rm opt}=3.1\times 10^{-5}$ at 300K.

###Synergetic enhancement of power factor and suppression of lattice thermal conductivity via electronic structure modification and nanostructuring on Ni and B co-doped p-type Si-Ge alloy|Muthusamy Omprakash,Saurabh Singh,Keisuke Hirata,Kentaro Kuga,Santhanakrishnan Harish,Masaru Shimomura,Masahiro Adachi,Yoshiyuki Yamamoto,Masaharu Matsunami,Tsunehiro Takeuchi###

Synergetic enhancement of power factor and suppression of lattice thermal conductivity via electronic structure modification and nanostructuring on Ni and B co-doped p-type Si-Ge alloy. For simultaneously achieving the high-power factor and low lattice thermal
conductivity of Si-Ge based thermoelectric materials, we employed, in this
study, constructively modifying the electronic structure near the chemical
potential and nano-structuring by low temperature and high-pressure sintering
on nano-crystalline powders. Nickel was doped to create the impurity states
near the edge of the valence band for enhancing the power factor with boron for
tuning the carrier concentration. The nanostructured samples with the nominal
composition of Si0.65-xGe0.32Ni0.03Bx (x = 0.01, 0.02, 0.03, and 0.04) were
synthesized by the mechanical alloying followed low-temperature and
high-pressure sintering process. A large magnitude of Seebeck coefficient
reaching 321 {\mu}VK-1 together with a small electrical resistivity of 4.49
m{\Omega}cm, leads to a large power factor of 2.3 Wm-1K-2 at 1000 K. With
successfully reduced thermal conductivity down to 1.47 Wm-1K-1, a large value
of ZT ~1.56 was obtained for Si0.65-xGe0.32Ni0.03B0.03 at 1000 K

###Efficient thermoelectricity in Sr$_2$Nb$_2$O$_7$ with energy-dependent relaxation times|Giulio Casu,Andrea Bosin,Vincenzo Fiorentini###

Efficient thermoelectricity in Sr$_2$Nb$_2$O$_7$ with energy-dependent relaxation times. We evaluate theoretically the thermoelectric efficiency of the layered
perovskite Sr$_2$Nb$_2$O$_7$ via calculations of the electronic structure and
transport coefficients within density-functional theory and Bloch-Boltzmann
relaxation-time transport theory. The predicted figure-of-merit tensor $ZT$,
computed with energy-, chemical potential- and temperature-dependent relaxation
times, has one component increasing monotonically from around 0.4 at room
temperature to 2.4 at 1250 K at an optimal carrier density around
2$\times$10$^{20}$ cm$^{-3}$, while the other components are small. The Seebeck
coefficient is about 250 to 300 $\mu$V/K at optimal doping, and reaches 800
$\mu$V/K at lower doping. We provide a {\tt python} code implementing various
approximations to the energy-dependent relaxation time transport, which can be
used to address different systems with an appropriate choice of material
parameters.

###High Thermoelectric Performance and Defect Energetics of Multi-pocketed Full-Heusler Compounds|Junsoo Park,Yi Xia,Alex Ganose,Anubhav Jain,Vidvuds Ozolins###

High Thermoelectric Performance and Defect Energetics of Multi-pocketed Full-Heusler Compounds. We report first-principles density-functional study of electron-phonon
interactions and thermoelectric transport properties of full-Heusler compounds
Sr$_{2}$BiAu and Sr$_{2}$SbAu. Our results show that ultrahigh intrinsic bulk
thermoelectric performance across a wide range of temperatures is physically
possible and point to the presence of multiply degenerate and highly dispersive
carrier pockets as the key factor for achieving it. Sr$_{2}$BiAu, which
features ten energy-aligned low effective mass pockets (six along $\Gamma-X$
and four at $L$), is predicted to deliver $n$-type $zT=0.4-4.9$ at
$T=100-700$~K. Comparison with the previously investigated Ba$_{2}$BiAu
compound shows that the additional $L$-pockets in Sr$_{2}$BiAu significantly
increase its low-temperature power factor to a maximum value of
$12$~mW~m$^{-1}$~K$^{-2}$ near $T=300$~K. However, at high temperatures the
power factor of Sr$_{2}$BiAu drops below that of Ba$_{2}$BiAu because the $L$
states are heavier and subject to strong scattering by phonon deformation as
opposed to the lighter $\Gamma-X$ states that are limited by polar-optical
scattering. Sr$_{2}$SbAu is predicted to deliver lower $n$-type of $zT=3.4$ at
$T=750$~K due to appreciable misalignment between the $L$ and $\Gamma-X$
carrier pockets, generally heavier scattering, and slightly higher lattice
thermal conductivity. Soft acoustic modes, responsible for low lattice thermal
conductivity, also increase vibrational entropies and high-temperature
stability of the Heusler compounds, suggesting that their experimental
synthesis may be feasible. The dominant intrinsic defects are found to be Au
vacancies, which drive the Fermi level towards the conduction band and work in
favor of $n$-doping.

###First-principles electronic structure, phonon properties, lattice thermal conductivity and prediction of figure of merit of FeVSb half-Heusler|Shivprasad S. Shastri,Sudhir K. Pandey###

First-principles electronic structure, phonon properties, lattice thermal conductivity and prediction of figure of merit of FeVSb half-Heusler. In this work, we have studied the electronic structure of a promising
thermoelectric half-Heusler FeVSb using FP-LAPW method and SCAN meta-GGA
including spin-orbit coupling. Using the obtained electronic structure and
transport calculations we try to address the experimental Seebeck coefficient
$S$ of FeVSb samples. The good agreement between the experimental and
calculated $S$ suggests the band gap could be $\sim$0.7 eV. This is supported
by the obtained mBJ band gap of $\sim$0.7 eV. Further, we study and report the
phonon dispersion, density of states and thermodynamic properties. The effect
of long range Coulomb interactions on phonon frequencies are also included by
non-analytical term correction. Under quasi-harmonic approximation, the thermal
expansion behaviour upto 1200 K is calculated. Using the first-principles
anharmonic phonon calculations, the lattice thermal conductivity $\kappa_{ph}$
of FeVSb is obtained under single-mode relaxation time approximation
considering the phonon-phonon interaction. At 300 K, the calculated
$\kappa_{ph}$ is $\sim$18.6 W$m^{-1}K^{-1}$ which is higher compared to
experimental value. But, above 500 K the calculated $\kappa_{ph}$ is in good
agreement with experiment. A prediction of figure of merit $ZT$ and efficiency
for p-type and n-type FeVSb is made by finding out optimal carrier
concentration. At 1200 K, a maximum $ZT$ of $\sim$0.66 and $\sim$0.44 is
expected for p-type and n-type FeVSb, respectively. For p-type and n-type
materials, maximum efficiency of $\sim$12.2 \% and $\sim$6.0 \% are estimated
for hot and cold temperature of 1200 K and 300 K, respectively. A possibility
of achieving n-type and p-type FeVSb by various elemental doping/vacancy is
also discussed. Our study is expected to help in further exploring the
thermoelectric material FeVSb.

###Hierarchically nanostructured thermoelectric materials: Challenges and opportunities for improved power factors|Neophytos Neophytou,Vassilios Vargiamidis,Samuel Foster,Patrizio Graziosi,Laura de Sousa Oliveira,Dhritiman Chakraborty,Zhen Li,Mischa Thesberg,Hans Kosina,Nick Bennett,Giovanni Pennelli,Dario Narducci###

Hierarchically nanostructured thermoelectric materials: Challenges and opportunities for improved power factors. The field of thermoelectric materials has undergone a revolutionary
transformation over the last couple of decades as a result of the ability to
nanostructure and synthesize myriads of materials and their alloys. The ZT
figure of merit, which quantifies the performance of a thermoelectric material
has more than doubled after decades of inactivity, reaching values larger than
two, consistently across materials and temperatures. Central to this ZT
improvement is the drastic reduction in the material thermal conductivity due
to the scattering of phonons on the numerous interfaces, boundaries,
dislocations, point defects, phases, etc., which are purposely included. In
these new generation of nanostructured materials, phonon scattering centers of
different sizes and geometrical configurations (atomic, nano- and macro-scale)
are formed, which are able to scatter phonons of mean-free-paths across the
spectrum. Beyond thermal conductivity reductions, ideas are beginning to emerge
on how to use similar hierarchical nanostructuring to achieve power factor
improvements. Ways that relax the adverse interdependence of the electrical
conductivity and Seebeck coefficient are targeted, which allows power factor
improvements. For this, elegant designs are required, that utilize for instance
non-uniformities in the underlying nanostructured geometry, non-uniformities in
the dopant distribution, or potential barriers that form at boundaries between
materials. A few recent reports, both theoretical and experimental, indicate
that extremely high power factor values can be achieved, even for the same
geometries that also provide ultra-low thermal conductivities. Despite the
experimental complications that can arise in having the required control in
nanostructure realization, in this colloquium, we aim to demonstrate, mostly
theoretically, that it is a very promising path worth exploring.

###High thermoelectric performance in metallic NiAu alloys|Fabian Garmroudi,Michael Parzer,Alexander Riss,Cédric Bourgès,Sergii Khmelevskyi,Takao Mori,Ernst Bauer,Andrej Pustogow###

High thermoelectric performance in metallic NiAu alloys. Thermoelectric (TE) materials seamlessly convert thermal into electrical
energy and vice versa, making them promising for applications such as power
generation or cooling. Although historically the TE effect was first discovered
in metals, state-of-the-art research mainly focuses on doped semiconductors
with large figure of merit, $zT$, that determines the conversion efficiency of
TE devices. While metallic alloys have superior functional properties, such as
high ductility and mechanical strength, they have mostly been discarded from
investigation in the past due to their small Seebeck effect. Here, we realize
unprecedented TE performance in metals by tuning the energy-dependent
electronic scattering. Based on our theoretical predictions, we identify binary
NiAu alloys as promising candidate materials and experimentally discover
colossal power factors up to 34 mWm$^{-1}$K$^{-2}$ (on average 30
mWm$^{-1}$K$^{-2}$ from 300 to 1100 K), which is more than twice larger than in
any known bulk material above room temperature. This system reaches a $zT$ up
to 0.5, setting a new world record value for metals. NiAu alloys are not only
orders of magnitude more conductive than heavily doped semiconductors, but also
have large Seebeck coefficients originating from an inherently different
physical mechanism: within the Au s band conduction electrons are highly mobile
while holes are scattered into more localized Ni d states, yielding a strongly
energy-dependent carrier mobility. Our work challenges the common belief that
good metals are bad thermoelectrics and presents an auspicious paradigm for
achieving high TE performance in metallic alloys through engineering
electron-hole selective s-d scattering.

###Theoretical insight into the thermoelectric behavior of tri-nuclear metal-string complexes laced with gold nanoelectrodes: A first-principles study|Talem Rebeda Roy,Arijit Sen###

Theoretical insight into the thermoelectric behavior of tri-nuclear metal-string complexes laced with gold nanoelectrodes: A first-principles study. Metal-string complexes in the quasi-1D framework may play an important role
in molecular electronics by serving not only as nanoscale interconnects but
also as active functional elements for nanoelectronic devices. However, because
of the potential volumetric heat generation across such nanojunctions, the
circuit stability becomes often a major concern, which necessitates to study
the heat transport properties at the molecular-scale. Here we report the
thermoelectric behavior of various tr-nuclear metal-string complexes,
$[M-M-M](dpa)_4(NCS)_2$ for $M \in \{Cr,Ru\}$, bridging Au(111) nanowires as
nanoelectrodes. Based on our charge transport analysis from
\textit{first-principles}, we find that the dominant transmission peaks tend to
move away from the Fermi level upon systematic rutheniation in chromium-based
metal-string complexes due mainly to the coupling of $\pi^{*}$ orbitals from Ru
and $\sigma_{nb}$ orbitals from Cr. Such type of a metal-string junction can
also exhibit strong Coulomb interaction so that its thermoelectric behavior
begins to deviate from the Wiedemann-Franz law. Our results further suggest
that metal-string complexes can render better thermoelectric devices especially
at the molecular-scale with the thermopower as high as 172 $\mu V/K$ at 300 K.
Considering the contributions from both electrons and phonons, even a high
\textit{figure of merit} of $ZT \sim 2$ may be attained for Cr-Cr-Cr based
metal-string molecular junctions at room temperature. Resonant enhancement in
the thermoelectric efficiency appears to occur in such systems through
alteration of inter-dot electrostatic interactions, which can be controlled by
incorporating Cr and Ru atoms in such tri-nuclear metal-string complexes.

###Thermoelectric efficiency at maximum power in low-dimensional systems|Natthapon Nakpathomkun,Hongqi Q. Xu,Heiner Linke###

Thermoelectric efficiency at maximum power in low-dimensional systems. Low-dimensional electronic systems in thermoelectrics have the potential to
achieve high thermal-to-electric energy conversion efficiency. A key measure of
performance is the efficiency when the device is operated under maximum power
conditions. Here we study the efficiency at maximum power of three
low-dimensional, thermoelectric systems: a zero-dimensional quantum dot (QD)
with a Lorentzian transmission resonance of finite width, a one-dimensional
(1D) ballistic conductor, and a thermionic (TI) power generator formed by a
two-dimensional energy barrier. In all three systems, the efficiency at maximum
power is independent of temperature, and in each case a careful tuning of
relevant energies is required to achieve maximal performance. We find that
quantum dots perform relatively poorly under maximum power conditions, with
relatively low efficiency and small power throughput. Ideal one-dimensional
conductors offer the highest efficiency at maximum power (36% of the Carnot
efficiency). Whether 1D or TI systems achieve the larger maximum power output
depends on temperature and area filling factor. These results are also
discussed in the context of the traditional figure of merit $ZT$.

###Mexican Hat and Rashba Bands in Few-Layer van der Waals Materials|Darshana Wickramaratne,Ferdows Zahid,Roger K. Lake###

Mexican Hat and Rashba Bands in Few-Layer van der Waals Materials. The valence band of a variety of few-layer, two-dimensional materials
consists of a ring of states in the Brillouin zone. The energy-momentum
relation has the form of a `Mexican hat' or a Rashba dispersion. The
two-dimensional density of states is singular at or near the band edge, and the
band-edge density of modes turns on nearly abruptly as a step function. The
large band-edge density of modes enhances the Seebeck coefficient, the power
factor, and the thermoelectric figure of merit ZT. Electronic and
thermoelectric properties are determined from ab initio calculations for
few-layer III-VI materials GaS, GaSe, InS, InSe, for Bi$_{2}$Se$_{3}$, for
monolayer Bi, and for bilayer graphene as a function of vertical field. The
effect of interlayer coupling on these properties in few-layer III-VI materials
and Bi$_{2}$Se$_{3}$ is described. Analytical models provide insight into the
layer dependent trends that are relatively consistent for all of these
few-layer materials. Vertically biased bilayer graphene could serve as an
experimental test-bed for measuring these effects.

###Enhanced thermoelectric figure-of-merit in boron-doped SiGe thin films by nanograin boundaries|Jianbiao Lu,Ruiqiang Guo,Weijing Dai,Baoling Huang###

Enhanced thermoelectric figure-of-merit in boron-doped SiGe thin films by nanograin boundaries. Boron-doped polycrystalline silicon-germanium (SiGe) thin films are grown by
low-pressure chemical vapor deposition (LPCVD) and their thermoelectric
properties are characterized from 120 K to 300 K for the potential applications
in integrated microscale cooling. The naturally formed grain boundaries are
found to play a crucial role in determining both the charge and thermal
transport properties of the films. Particularly, the unique columnar grain
structures result in remarkable thermal conductivity anisotropy with the
in-plane thermal conductivities of SiGe films about 50% lower than the
cross-plane values. By optimizing the growth conditions and doping level, a
high figure of merit (ZT) of 0.2 for SiGe films is achieved at 300 K, which is
about 100% higher than the previous record for p-type SiGe alloys, mainly due
to the significant reduction in the in-plane thermal conductivity caused by
nanograin boundaries. The low cost and excellent scalability of LPCVD render
these high-performance SiGe films ideal candidates for thin-film thermoelectric
applications.

###Understanding the role and interplay of heavy hole and light hole valence bands in the thermoelectric properties of PbSe|Thomas C. Chasapis,Yeseul Lee,Euripides Hatzikraniotis,Konstantinos M. Paraskevopoulos,Hang Chi,Ctirad Uher,Mercouri G. Kanatzidis###

Understanding the role and interplay of heavy hole and light hole valence bands in the thermoelectric properties of PbSe. The thermoelectric properties of PbSe have significantly improved in recent
years reaching figures of merit ZT 1.6. The transport properties of the hole
doped high temperature thermoelectric material PbSe are particularly
interesting and play a key role in this. Here they were analyzed over a wide
temperature and hole concentration ranges. The special features observed in the
variation of the experimental Seebeck coefficient, and Hall coefficient can be
accounted for within the framework of a two band model. Two valence bands
separated by a temperature dependent energy offset are considered. The extremum
of the light hole band has a density of states mass 0.27mo at room temperature.
It is non-parabolic and anisotropic and can be described by the Kane model. The
extremum of the heavy hole band is isotropic and parabolic with a much larger
density of states mass 2.5mo. We find that for heavily doped compositions the
high mass band contributes the Seebeck coefficient even at room temperature.
With rising temperature holes are transferred from the light hole to the heavy
hole branch giving rise to the anomalous temperature dependent Hall coefficient
which is found peaked near 650 K.

###Thermoelectric properties of a ferromagnet-superconductor hybrid junction: Role of interfacial Rashba spin-orbit interaction|Paramita Dutta,Arijit Saha,A. M. Jayannavar###

Thermoelectric properties of a ferromagnet-superconductor hybrid junction: Role of interfacial Rashba spin-orbit interaction. We investigate thermoelectric properties of a ferromagnet-superconductor
hybrid structure with Rashba spin-orbit interaction and delta function
potential barrier at the interfacial layer. The exponential rise of thermal
conductance with temperature manifests a cross-over temperature scale
separating two opposite behaviors of it with the change of polarization in the
ferromagnet whereas the inclusion of interfacial Rashba spin-orbit field
results in a non-monotonic behavior of it with the strength of Rashba field. We
employ scattering matrix approach to determine the amplitudes of all the
scattering processes possible at the interface to explain the thermoelectric
properties of the device. We examine Seebeck effect and show that higher
thermopower can be achieved when the polarization of the ferromagnet tends
towards the half-metallic limit. It can be enhanced even for lower polarization
in presence of the finite potential barrier. In presence of interfacial Rashba
spin-orbit interaction, Seebeck coefficient rises with the increase of barrier
strength and polarization at weak or moderate interfacial Rashba field. From
the application perspective, we compute the figure of merit and show that
$zT\sim 4-5$ with higher polarization of the ferromagnet both in absence and
presence of weak or moderate Rashba spin-orbit interaction along with the
scalar potential barrier.

###Thermoelectric properties of an interacting quantum dot-based heat engine|Paolo Andrea Erdman,Francesco Mazza,Riccardo Bosisio,Giuliano Benenti,Rosario Fazio,Fabio Taddei###

Thermoelectric properties of an interacting quantum dot-based heat engine. We study the thermoelectric properties and heat-to-work conversion
performance of an interacting, multi-level quantum dot (QD) weakly coupled to
electronic reservoirs. We focus on the sequential tunneling regime. The
dynamics of the charge in the QD is studied by means of master equations for
the probabilities of occupation. From here we compute the charge and heat
currents in the linear response regime. Assuming a generic multi-terminal
setup, and for low temperatures (quantum limit), we obtain analytical
expressions for the transport coefficients which account for the interplay
between interactions (charging energy) and level quantization. In the case of
systems with two and three terminals we derive formulas for the power factor Q
and the figure of merit ZT for a QD-based heat engine, identifying optimal
working conditions which maximize output power and efficiency of heat-to-work
conversion. Beyond the linear response we concentrate on the two-terminal
setup. We first study the thermoelectric non-linear coefficients assessing the
consequences of large temperature and voltage biases, focusing on the breakdown
of the Onsager reciprocal relation between thermopower and Peltier coefficient.
We then investigate the conditions which optimize the performance of a heat
engine, finding that in the quantum limit output power and efficiency at
maximum power can almost be simultaneously maximized by choosing appropriate
values of electrochemical potential and bias voltage. At last we study how
energy level degeneracy can increase the output power.

###Dynamic surface electronic reconstruction as symmetry-protected topological orders in topological insulator Bi2Se3|G. J. Shu,S. C. Liou,S. K. Karna,R. Sankar,M. Hayashi,F. C. Chou###

Dynamic surface electronic reconstruction as symmetry-protected topological orders in topological insulator Bi2Se3. Layered narrow band gap semiconductor Bi2Se3 is composed of heavy elements
with strong spin-orbital coupling (SOC), which has been identified both as a
good candidate of thermoelectric material of high thermoelectric
figure-of-merit (ZT) and a topological insulator of Z2-type with a gapless
surface band in Dirac cone shape. The existence of a conjugated pi-bond system
on the surface of each Bi2Se3 quintuple layer is proposed based on an extended
valence bond model having valence electrons distributed in the hybridized
orbitals. Supporting experimental evidences of a 2D conjugated pi-bond system
on each quintuple layer of Bi2Se3 are provided by electron energy-loss
spectroscopy (EELS) and electron density (ED) mapping through inverse Fourier
transform of X-ray diffraction data. Quantum chemistry calculations support the
pi-bond existence between partially filled 4pz orbitals of Se via side-to-side
orbital overlap positively. The conjugated pi-bond system on the surface of
each quintuple Bi2Se3 layer is proposed being similar to that found in graphite
(graphene) and responsible for the unique 2D conduction mechanism. The van der
Waals (vdW) attractive force between quintuple layers is interpreted being
coming from the anti-ferroelectrically ordered effective electric dipoles which
are constructed with pi-bond trimer pairs on Se-layers across the vdW gap of
minimized Coulomb repulsion.

###Thermoelectric Properties of Polycrystalline NiSi3P4|Andrew F. May,Michael A. McGuire,Hsin Wang###

Thermoelectric Properties of Polycrystalline NiSi3P4. The Hall and Seebeck coefficients, electrical resistivity and thermal
conductivity of polycrystalline NiSi3P4 were characterized from 2 to 775K.
Undoped NiSi3P4 behaves like a narrow gap semiconductor, with activated
electrical resistivity \rho below room temperature and a large Seebeck
coefficient of ~400uV/K at 300K. Attempts to substitute boron for silicon
resulted in the production of extrinsic holes, yielding moderately-doped
semiconductor behavior with \rho increasing with increasing temperature above
~150\,K. Hall carrier densities are limited to approximately 5x10^{19}/cm^3 at
200K, which would suggest the solubility limit of boron is reached if boron is
indeed incorporated into the lattice. These extrinsic samples have a Hall
mobility of ~12cm^2/V/s at 300K, and a parabolic band equivalent effective mass
of ~3.5 times the free electron mass. At 700,K, the thermoelectric figure of
merit zT reaches ~0.1. Further improvements in thermoelectric performance would
require reaching higher carrier densities, as well as a mechanism to further
reduce the lattice thermal conductivity, which is ~5W/m/K at 700K. Alloying in
Ge results in a slight reduction of the thermal conductivity at low
temperatures, with little influence observed at higher temperatures.

###The influence of non-idealities on the thermoelectric power factor of nanostructured superlattices|Mischa Thesberg,Mahdi Pourfath,Hans Kosina,Neophytos Neophytou###

The influence of non-idealities on the thermoelectric power factor of nanostructured superlattices. Cross-plane superlattices composed of nanoscale layers of alternating
potential wells and barriers have attracted great attention for their potential
to provide thermoelectric power factor improvements and higher ZT figure of
merit. Previous theoretical works have shown that the presence of optimized
potential barriers could provide improvements to the Seebeck coefficient
through carrier energy filtering, which improves the power factor by up to 40%.
However, experimental corroboration of this prediction has been extremely
scant. In this work, we employ quantum mechanical electronic transport
simulations to outline the detrimental effects of random variation,
imperfections and nonoptimal barrier shapes in a superlattice geometry on these
predicted power factor improvements. Thus we aim to assess either the
robustness or the fragility of these theoretical gains in the face of the types
of variation one would find in real material systems. We show that these power
factor improvements are relatively robust against: overly thick barriers,
diffusion of barriers into the body of the wells, and random fluctuations in
barrier spacing and width. However, notably, we discover that extremely thin
barriers and random fluctuation in barrier heights by as little as 10% is
sufficient to entirely destroy any power factor benefits of the optimized
geometry. Our results could provide performance optimization routes for
nanostructured thermoelectrics and elucidate the reasons why significant power
factor improvements are not commonly realized in superlattices, despite
theoretical predictions.

###Thermoelectric properties of graphene/boron nitride heterostructures|L. A. Algharagholy,Q. Al-Galiby,H. A. Marhoon,H. Sadeghi,H. M. Abduljalil,C. J. Lambert###

Thermoelectric properties of graphene/boron nitride heterostructures. Using density functional theory combined with a Green's function scattering
approach, we examine the thermoelectric properties of hetero-nanoribbons formed
from alternating lengths of graphene and boron nitride. In such structures, the
boron nitride acts as a tunnel barrier, which weakly couples states in the
graphene, to form mini-bands . In un-doped nanoribbons, the mini bands are
symmetrically positioned relative to the Fermi energy and do not enhance
thermoelectric performance significantly. In contrast, when the ribbons are
doped by electron donating or electron accepting adsorbates, the thermopower S
and electronic figure of merit are enhanced and either positive or negative
thermopowers can be obtained. In the most favourable case, doping with the
electron donor tetrathiafulvalene (TTF) increases the room-temperature
thermopower to -284 {\mu}v/K and doping by the electron acceptor
tetracyanoethylene (TCNE) increases S to 210 {\mu}v/K. After including both
electron and phonon contributions to the thermal conductance, figures of merit
ZT up to of order 0.9 are obtained.

###Oligoyne molecular junctions for efficient room temperature thermoelectric power generation|Hatef Sadeghi,Sara Sangtarash,Colin J. Lambert###

Oligoyne molecular junctions for efficient room temperature thermoelectric power generation. Understanding phonon transport at a molecular scale is fundamental to the
development of high-performance thermoelectric materials for the conversion of
waste heat into electricity. We have studied phonon and electron transport in
alkane and oligoyne chains of various lengths and find that due to the more
rigid nature of the latter, the phonon thermal conductances of oligoynes are
counter intuitively lower than that of the corresponding alkanes. The thermal
conductance of oligoynes decreases monotonically with increasing length,
whereas the thermal conductance of alkanes initially increases with length and
then decreases. This difference in behaviour arises from phonon filtering by
the gold electrodes and disappears when higher-Debye-frequency electrodes are
used. Consequently a molecule that better transmits higher-frequency phonon
modes, combined with a low-Debye-frequency electrode that filters high-energy
phonons is a viable strategy for suppressing phonon transmission through the
molecular junctions. The low thermal conductance of oligoynes, combined with
their higher thermopower and higher electrical conductance lead to yield a
maximum thermoelectric figure of merit of ZT = 1.4, which is several orders of
magnitude higher than for alkanes.

###Large-scale molecular dynamics investigation of geometrical features in nanoporous Si|Laura de Sousa Oliveira,Neophytos Neophytou###

Large-scale molecular dynamics investigation of geometrical features in nanoporous Si. Nanoporous materials are of broad interest for various applications, in
particular advanced thermoelectric materials. The introduction of nanoscale
porosity, even at modest levels, has been known to drastically reduce a
materials thermal conductivity, in some cases even below its amorphous limit,
thereby significantly increasing its thermoelectric figure of merit, ZT. The
details of the important attributes that drive these large reductions, however,
are not yet clear. In this work, we employ large-scale equilibrium molecular
dynamics to perform an exhaustive atomistic-scale investigation of the effect
of porosity on thermal transport in nanoporous bulk silicon. Thermal transport
is computed for over 50 different geometries, spanning a large number of
geometrical degrees of freedom, such as cylindrical pores and voids, different
porosities, diameters, neck sizes, pore/void numbers, and surface-to-volume
ratios, placed in ordered fashion, or fully disordered. We thus quantify and
compare the most important parameters that determine the thermal conductivity
reductions in nanoporous materials. Ultimately, we find that, even at the
nanoscale, the effect of merely reducing the line-of-sight of phonons, i.e. the
clear pathways that phonons can utilize during transport, plays the most
crucial role in reducing the thermal conductivity in nanoporous materials,
beyond other metrics such as porosity and surface/boundary scattering.

###Modelling thermoelectric performance in nanoporous nanocrystalline silicon|Laura de Sousa Oliveira,Vassilios Vargiamidis,Neophytos Neophytou###

Modelling thermoelectric performance in nanoporous nanocrystalline silicon. Introducing hierarchical disorder from multiple defects into materials
through nanostructuring is one of the most promising directions to achieve
extremely low thermal conductivities and thus improve thermoelectric
performance. The success of nanostructuring relies on charge carriers having
shorter mean-free-paths than phonons so that the latter can be selectively
scattered. Nevertheless, introducing disorder into a material often comes at
the expense of scattering charge carriers as well as phonons. In order to
determine the tradeoff between the degradation of the lattice thermal
conductivity and of the power factor due to this, we perform a theoretical
investigation of both phonon and electron transport in nanocrystalline,
nanoporous Si geometries. We use molecular dynamics for phonon transport
calculations and the non-equilibrium Green's function method for electronic
transport. We report on the engineering tradeoff that the porosity (number of
pores and their in-between distance) has on the overall thermoelectric
performance for the material optimization. We indeed find that the reduction in
thermal conductivity is stronger compared to the reduction in the power factor,
for the low porosities considered in this study (up to 5 %), and that the ZT
figure of merit can experience a large increase, especially when grain
boundaries are included, compared to just nanoporosity.

###Lattice thermal transport in group II-alloyed PbTe|Yi Xia,James M. Hodges,Mercouri G. Kanatzidis,Maria K. Y. Chan###

Lattice thermal transport in group II-alloyed PbTe. PbTe, one of the most promising thermoelectric materials, has recently
demonstrated thermoelectric figure of merit ($ZT$) of above 2.0 when alloyed
with group II elements. The improvements are due mainly to significant
reduction of lattice thermal conductivity ($\kappa_{l}$), which was in turn
attributed to nanoparticle precipitates. However, a fundamental understanding
of various phonon scattering mechanisms within the bulk alloy is still lacking.
In this work, we apply the newly-developed density-functional-theory
(DFT)-based compressive sensing lattice dynamics (CSLD) approach to model
lattice heat transport in PbTe, MTe, and Pb$_{0.94}$M$_{0.06}$Te (M=Mg, Ca, Sr
and Ba), compare our results with experimental measurements, with focus on
strain effect and mass disorder scattering. We find that (1) CaTe, SrTe and
BaTe in the rock-salt structure exhibit much higher $\kappa_{l}$ than PbTe,
while MgTe in the same structure shows anomalously low $\kappa_{l}$; (2)
lattice heat transport of PbTe is extremely sensitive to static strain induced
by alloying atoms in solid solution form; (3) mass disorder scattering plays a
major role in reducing $\kappa_{l}$ for Mg/Ca/Sr-alloyed PbTe through strongly
suppressing the lifetimes of intermediate- and high-frequency phonons, while
for Ba-alloyed PbTe, precipitated nanoparticles are also important.

###First-principles study of the layered thermoelectric material TiNBr|Shuofeng Zhang,Ben Xu,Yuanhua Lin,Cewen Nan,Wei Liu###

First-principles study of the layered thermoelectric material TiNBr. Layer-structured materials are often considered to be good candidates for
thermoelectric materials, because they tend to exhibit intrinsically low
thermal conductivity as a result of atomic interlayer interactions. The
electrical properties of layer-structured materials can be easily tuned using
various methods, such as band modification and intercalation. We report TiNBr,
as a member of the layer-structured metal nitride halide system MNX (M = Ti,
Zr, Hf; X = Cl, Br, I), and it exhibits an ultrahigh Seebeck coefficient of
2215 $\mu V/K$ at 300K. The value of the dimensionless figure of merit, ZT,
along A axis can be as high as 0.661 at 800K, corresponding to a lattice
thermal conductivity as low as 1.34 W/(m K). The low ${\kappa_l}$ of TiNBr is
associated with a collectively low phonon group velocity ($2.05\times 10^3 $
m/s on average) and large phonon anharmonicity that can be quantified using the
Gr\"uneisen parameter and three-phonon processes. Animation of the atomic
motion in highly anharmonic modes mainly involves the motion of N atoms, and
the charge density difference reveals that the N atoms become polarized with
the merging of anharmonicity. Moreover, the fitting procedure of the
energy-displacement curve verifies that in addition to the three-phonon
processes, the fourth-order anharmonic effect is also important in the integral
anharmonicity of TiNBr. Our work is the first study of the thermoelectric
properties of TiNBr and may help establish a connection between the low lattice
thermal conductivity and the behavior of phonon vibrational modes.

###Thermoelectric phonon glass electron crystal via ion beam patterning of silicon|Taishan Zhu,Krishnan Swaminathan-Gopalan,Kelly Stephani,Elif Ertekin###

Thermoelectric phonon glass electron crystal via ion beam patterning of silicon. Ion beam irradiation has recently emerged as a versatile approach to
functional materials design. We show in this work that patterned defective
regions generated by ion beam irradiation of silicon can create a phonon glass
electron crystal (PGEC), a longstanding goal of thermoelectrics. By controlling
the effective diameter of and spacing between the defective regions, molecular
dynamics simulations suggest a reduction of the thermal conductivity by a
factor of $\approx$20 is achievable. Boltzmann theory shows that the
thermoelectric power factor remains largely intact in the damaged material. To
facilitate the Boltzmann theory, we derive an analytical model for electron
scattering with cylindrical defective regions based on partial wave analysis.
Together we predict a figure of merit of ZT$\approx$0.5 or more at room
temperature for optimally patterned geometries of these silicon metamaterials.
These findings indicate that nanostructuring of patterned defective regions in
crystalline materials is a viable approach to realize a PGEC, and ion beam
irradiation could be a promising fabrication strategy.

###Analytic treatment of the thermoelectric properties for two coupled quantum dots threaded by magnetic fields|Guido Menichetti,Giuseppe Grosso,Giuseppe Pastori Parravicini###

Analytic treatment of the thermoelectric properties for two coupled quantum dots threaded by magnetic fields. Coupled double quantum dots (c-2QD) connected to leads have been widely
adopted as prototype model systems to verify interference effects on quantum
transport at the nanoscale. We provide here an analytic study of the
thermoelectric properties of c-2QD systems pierced by a uniform magnetic field.
Fully analytic and easy-to-use expressions are derived for all the kinetic
functionals of interest. Within the Green 0 s function formalism, our results
allow a simple inexpensive procedure for the theoretical description of the
thermoelectric phenomena for different chemical potentials and temperatures of
the reservoirs, different threading magnetic fluxes, dot energies and interdot
interactions; moreover they provide an intuitive guide to parametrize the
system Hamiltonian for the design of best performing realistic devices. We have
found that the thermopower S can be enhanced by more than ten times and the
figure of merit ZT by more than hundred times by the presence of a threading
magnetic field. Most important, we show that the magnetic flux increases also
the performance of the device under maximum power output conditions.

###Thermoelectric properties of gapped bilayer graphene|Dominik Suszalski,Grzegorz Rut,Adam Rycerz###

Thermoelectric properties of gapped bilayer graphene. Unlike in conventional semiconductors, both the chemical potential and the
band gap in bilayer graphene (BLG) can be tuned via application of external
electric field. Among numerous device implications, this property also
designates BLG as a candidate for high-performance thermoelectric material. In
this theoretical study we have calculated the Seebeck coefficients for abrupt
interface separating weakly- and heavily-doped areas in BLG, and for a more
realistic rectangular sample of mesoscopic size, contacted by two electrodes.
For a given band gap ($\Delta$) and temperature ($T$) the maximal Seebeck
coefficient is close to the Goldsmid-Sharp value $|S|_{\rm max}^{\rm
GS}=\Delta/(2eT)$, the deviations can be approximated by the asymptotic
expression $|S|_{\rm max}^{\rm GS}-|S|_{\rm
max}=(k_B/e)\times\left[\frac{1}{2}\ln{u}+\ln{}2-\frac{1}{2}+{\cal
O}(u^{-1})\right]$, with the electron charge $-e$, the Boltzmann constant
$k_B$, and $u = \Delta/(2k_BT)\gg{}1$. Surprisingly, the effects of trigonal
warping term in the BLG low-energy Hamiltonian are clearly visible at
few-Kelvin temperatures, for all accessible values of
$\Delta\leqslant{}300\,$meV. We also show that thermoelectric figure of merit
is noticeably enhanced ($ZT>3$) when a rigid substrate suppresses out-of-plane
vibrations, reducing the contribution from $ZA$ phonons to the thermal
conductivity.

###Designing graphene/hexagonal boron nitride superlattice monolayer with high thermoelectric performance|Zizhen Zhou,Huijun Liu,Dengdong Fan,Guohua Cao###

Designing graphene/hexagonal boron nitride superlattice monolayer with high thermoelectric performance. We design a hybrid graphene/hexagonal boron nitride superlattice monolayer
and investigate its thermoelectric properties using density functional theory
and Boltzmann transport equations with the relaxation time accurately treated
by electron-phonon coupling calculations. Compared with that of pristine
graphene, the lattice thermal conductivity of the superlattice structure is
more than two orders of magnitude lower due to the enhanced three-phonon
scattering process originated from the mixed-bond characteristics. Besides, the
coexistence of light and heavy bands around the Fermi level leads to an
ultrahigh power factor along the zigzag direction, where the highest ZT value
of ~2.5 can be achieved for the n-type system at 1100 K. Moreover, it is noted
that the carrier transport near the valance band minimum is almost entirely
contributed by the graphene part of the superlattice. As a consequence, the
thermoelectric performance of p-type system can be enhanced to be comparable
with that of n-type one by appropriate substitution of nitrogen atom with
phosphorus, which can suppress the lattice thermal conductivity but nearly have
no influence on the hole transport.

###High Performance Ternary Alkali Nitrides for Renewable Energy Applications|Jiban Kangsabanik,Aftab Alam###

High Performance Ternary Alkali Nitrides for Renewable Energy Applications. Rapid decline in fossil fuel energy necessitates the immediate need for
renewable energy resources. Here, we report a previously unexplored class of
nitrides AMN$_2$ keeping renewable energy applications in mind. Using a
detailed structure and stability analysis using first principles simulation, we
discovered twelve such compounds (few of which are already synthesized before),
which are chemically, mechanically and dynamically stable. These twelve
compounds were then evaluated for their suitability for three renewable energy
applications, (i) photovoltaics, (ii) water splitting, and (iii)
thermoelectrics. Careful analysis of electronic structure reveals high optical
transition strength resulting in sharp rise in absorption. This in turn yields
high short circuit current and hence excellent solar efficiency for few
compounds namely CsVN$_2$ and RbVN$_2$. Along with excellent absorption
quality, some compounds show favorable band edge positions compared to water
redox levels and hence are promising as photoelectrodes in
photo(electro)chemical water splitting devices. Mixture of flat and dispersive
bands in the band structure yields both high Seebeck and electrical
conductivity, thus excellent power factor for seven compounds. Simulated
lattice thermal conductivity shows moderate to ultralow values and thus the
possibility of achieving high thermoelectric figure of merit (ZT), even at
lower temperatures. From the experimental perspective, we discuss the possible
challenges that may arise while utilizing these compounds for the desired
applications, and suggest possible pathways to overcome them. We believe such
theoretical prediction of promising materials are extremely useful for new
materials discovery and anticipate rapid response from the experimental
community.

###Photo-Induced Ultrafast Symmetry Switch in SnSe|Yadong Han,Junhong Yu,Hang Zhang,Fang Xu,Kunlin Peng,Xiaoyuan Zhou,Liang Qiao,Oleg V. Misochko,Kazutaka G. Nakamura,Giovanni M. Vanacore,Jianbo Hu###

Photo-Induced Ultrafast Symmetry Switch in SnSe. Layered tin selenide (SnSe) has recently emerged as a high-performance
thermoelectric material with the current record for the figure of merit (ZT)
observed in the high-temperature Cmcm phase. So far, access of the Cmcm phase
has been mainly obtained via thermal equilibrium methods based on sample
heating or application of external pressure, thus restricting the current
understanding only to ground-state conditions. Here, we investigate the
ultrafast carrier and phononic dynamics in SnSe. Our results demonstrate that
optical excitations can transiently switch the point-group symmetry of the
crystal from Pnma to Cmcm at room temperature in a few hundreds of femtoseconds
with an ultralow threshold for the excitation carrier density. This
non-equilibrium Cmcm phase is found to be driven by the displacive excitation
of coherent Ag phonons and, given the absence of low-energy thermal phonons,
exists in SnSe with the status of 'cold lattice with hot carriers'. Our
findings provide important insight for understanding non-equilibrium
thermoelectric properties of SnSe.

###Optical phonon modes assisted thermal conductivity in p-type ZrIrSb Half-Heusler alloy: A combined experimental and computational study|Kavita Yadav,Saurabh Singh,Tsunehiro Takeuchi,K. Mukherjee###

Optical phonon modes assisted thermal conductivity in p-type ZrIrSb Half-Heusler alloy: A combined experimental and computational study. Half Heusler (HH) alloys with 18 valence electron count have attracted
significant interest in the area of research related to thermoelectrics.
Understanding the novel transport properties exhibited by these systems with
semiconducting ground state is an important focus area in this field. Large
thermal conductivity shown by most of the HH alloy possesses a major hurdle in
improving the figure of merit (ZT). Additionally, understanding the mechanism
of thermal conduction in heavy constituents HH alloys is an interesting aspect.
Here, we have investigated the high temperature thermoelectric properties of
ZrIrSb through experimental studies, phonon dispersion and electronic band
structure calculations. ZrIrSb is found to exhibit substantially lower
magnitude of resistivity and Seebeck coefficient near room temperature, owing
to existence of anti-site disorder between Ir/Sb and vacant sites.
Interestingly, in ZrIrSb, lattice thermal conductivity is governed by coupling
between the acoustic and low frequency optical phonon modes, which originates
due to heavier Ir/Sb atoms. This coupling leads to an enhancement in the
Umklapp processes due to the optical phonon excitations near zone boundary,
resulting in a lower magnitude of \k{appa}L. Our studies point to the fact that
the simultaneous existence of two heavy mass elements within a simple unit cell
can substantially decrease the lattice degrees of freedom.

###Temperature induced first order electronic topological transition in $β$-Ag$_2$Se|L. S. Sharath Chandra,SK. Ramjan,Soma Banik,Archna Sagdeo,M. K. Chattopadhyay###

Temperature induced first order electronic topological transition in $β$-Ag$_2$Se. $\beta$-Ag$_2$Se is a promising material for room temperature thermoelectric
applications and magneto-resistive sensors. However, no attention was paid
earlier to the hysteresis in the temperature dependence of resistivity
($\rho$($T$)). Here, we show that a broad hysteresis above 35 K is observed not
only in $\rho$($T$), but also in other electronic properties such as Hall
coefficient ($R_H$($T$)), Seebeck coefficient, thermal conductivity and
ultraviolet photoelectron spectra (UPS). We also show that the hysteresis is
not associated with a structural transition. The $\rho$($T$) and $R_H$($T$)
show that $\beta$-Ag$_2$Se is semiconducting above 300 K, but metallicity is
retained below 300 K. While electronic states are absent in the energy range
from the Fermi level ($E_F$) to 0.4 eV below the $E_F$ at 300 K, a distinct
Fermi edge is observed in the UPS at 15 K suggesting that the $\beta$-Ag$_2$Se
undergoes an electronic topological transition from a high temperature
semiconducting state to a low temperature metallic state. Our study reveals
that a constant and moderately high thermoelectric figure of merit ($ZT$) in
the range 300-395 K is observed due to the broad semiconductor to metal
transition in $\beta$-Ag$_2$Se.

###Improving the operational stability of thermoelectric Zn$_4$Sb$_3$ by segmentation|Peter Skjøtt Thorup,Christian Moeslund Zeuthen,Kasper Borup,Bo Brummerstedt Iversen###

Improving the operational stability of thermoelectric Zn$_4$Sb$_3$ by segmentation. The mixed ionic-electronic conductor $\beta$-Zn$_4$Sb$_3$ is a cheap and high
performing thermoelectric material, but under operating conditions with a
temperature gradient and a running current, the material decomposes as Zn
readily migrates in the structure. Here, we report an improved stability of
$\beta$-Zn$_4$Sb$_3$ by introducing ion-blocking interfaces of stainless steel
to segment the sample, produced by a rapid one-step Spark Plasma Sintering
synthesis. The stability of the samples is tested under temperature gradients
and electric currents, which reveals significantly improved stability of the
segmented samples compared to unsegmented samples. The segmented samples are
stable under temperature gradient from 250{\deg}C to room temperature with no
external current, whereas the unsegmented sample decomposes into ZnSb and Zn
under the same conditions. The thermoelectric figure of merit, zT, of the
segmented sample is slightly reduced, mainly due to the increased thermal
conductivity. In conclusion, a rapid one-step synthesis of segmented
$\beta$-Zn$_4$Sb$_3$ is developed, which successfully improves the long-term
operational stability by blocking the Zn ion migration.

###First Principles Study of the Structural, Mechanical, Electronic, and Lattice Dynamical Properties of the Half-Heusler Alloys ZrCoY (Y=Sb, Bi )|Lynet Allan,Winfred M. Mulwa,Robinson J. Musembi,Bernard O. Aduda###

First Principles Study of the Structural, Mechanical, Electronic, and Lattice Dynamical Properties of the Half-Heusler Alloys ZrCoY (Y=Sb, Bi ). First-principles calculation has led to significant discoveries in materials
science. Half heusler (HH) alloys, which are potential thermoelectric materials
have demonstrated significant improvements in thermoelectric performance owing
to their thermal stability, mechanical strength, and moderate ZT. Using Density
Functional Theory (DFT), the structural, mechanical, electronic, and lattice
dynamical properties of cubic Half Heusler alloys ZrCoY (Y=Sb, Bi) have been
investigated. The unknown exchange-correlation functional is approximated using
the generalized gradient approximation (GGA) pseudopotential plane-wave
approach. The structural parameters, that is, equilibrium lattice constant,
elastic constants, and their derivatives are consistent with reported
experimental and theoretical studies where available. Mechanical properties
such as anisotropy factor A, shear modulus G, bulk modulus B, Youngs modulus E,
and Poisons ratio n, are calculated using the Voigt-Reuss-Hill average approach
based on elastic constants. The Debyes temperature, as well as longitudinal and
transverse sound velocities, are predicted from elastic constants at GGA-PBE
and GW approximations. The study of elastic constants showed that the compounds
are mechanically stable, and the phonon dispersion study showed that they are
dynamically stable as well. The ductility and anisotropic nature of the
compounds are confirmed by the elastic constants and mechanical properties.

###High thermoelectric performances in PbP monolayers considering full electron-phonon coupling and four-phonon scattering processes|Ao Wu,Yiming Zhang,Yujie Xia,Lei Peng,Heyuan Zhu,Hezhu Shao,Hao Zhang###

High thermoelectric performances in PbP monolayers considering full electron-phonon coupling and four-phonon scattering processes. The band convergence strategy, which improves Seebeck coefficient by inducing
multi-valley in bandstructures, has been widely used in thermoelectric
performance (TE) enhancing. However, the phonon-assisted intervalley scattering
effect is neglected and the mode-selection rules remain unclear. In this work,
TE properties for $\alpha$-, $\beta$- and $\gamma$-PbP are intestigated under
the consideration of full mode-, energy- and momentum-resolved electron-phonon
interactions (EPI). The group theory is used to analyze the selection rules for
EPI matrix elements. Our calculations reveal that, the intervalley scattering
contributes non-trivially to the total carrier relaxation time, and the
intervalley scattering can be modulated through crystal symmetry. In addition,
the investigation on the thermal properties reveals that four-phonon scattering
effect dominates the phonon relaxation processes, since the three-phonon
scattering is suppressed due to the significantly large acoustic-optical phonon
bandgap in $\alpha$-, $\beta$- and $\gamma$-PbP. By considering full EPI effect
and high-order phonon scattering processes, the calculated ZT values reach
0.90, 0.24 and 1.25 for $\alpha$-, $\beta$- and $\gamma$-PbP, repectively,
indicating their promising applications in thermoelectric devices.

###Vacancy-Ordered Double Perovskites Cs$_2$BI$_6$ (B = Pt, Pd, Te, Sn): An Emerging Class of Thermoelectric Materials|Preeti Bhumla,Manjari Jain,Sajjan Sheoran,Saswata Bhattacharya###

Vacancy-Ordered Double Perovskites Cs$_2$BI$_6$ (B = Pt, Pd, Te, Sn): An Emerging Class of Thermoelectric Materials. Vacancy-ordered double perovskites (A$_2$BX$_6$), being one of the
environmentally friendly and stable alternatives to lead halide perovskites,
have garnered considerable research attention in the scientific community.
However, their thermal transport has not been explored much despite their
potential applications. Here, we explore Cs$_2$BI$_6$ (B = Pt, Pd, Te, Sn) as
potential thermoelectric materials using the state-of-the-art first-principles
based methodologies, viz., density functional theory combined with many-body
perturbation theory (G$_0$W$_0$) and spin-orbit coupling. %The phonon
dispersion plots and Poisson's and Pugh's ratios show the dynamical and
mechanical stability of this class of perovskites. The absence of polyhedral
connectivity in vacancy-ordered perovskites gives rise to additional degrees of
freedom leading to lattice anharmonicity. The presence of anharmonic lattice
dynamics leads to strong electron-phonon coupling, which is well captured by
Fr\"{o}hlich mesoscopic model. % to investigate the interaction of longitudinal
optical phonon modes with the carriers that strongly influence the carrier
mobility. The lattice anharmonicity is further studied using {\it ab initio}
molecular dynamics and electron localization function. The maximum
anharmonicity is observed in Cs$_2$PtI$_6$, followed by Cs$_2$PdI$_6$,
Cs$_2$TeI$_6$ and Cs$_2$SnI$_6$. Also, the computed average thermoelectric
figure of merit ($zT$) for Cs$_2$PtI$_6$, Cs$_2$PdI$_6$, Cs$_2$TeI$_6$ and
Cs$_2$SnI$_6$ are 0.88, 0.85, 0.95 and 0.78, respectively, which reveals their
promising renewable energy applications.

###Inorganic Tin Perovskites with Tunable Conductivity Enabled by Organic Modifiers|Md Azimul Haque,Tong Zhu,Luis Huerta Hernandez,Roba Tounesi,Craig Combe,Bambar Davaasuren,Abdul-Hamid Emwas,F. Pelayo García de Arquer,Edward H. Sargent,Derya Baran###

Inorganic Tin Perovskites with Tunable Conductivity Enabled by Organic Modifiers. Achieving control over the transport properties of charge-carriers is a
crucial aspect of realizing high-performance electronic materials. In
metal-halide perovskites, which offer convenient manufacturing traits and
tunability for certain optoelectronic applications, this is challenging: The
perovskite structure itself, poses fundamental limits to maximum dopant
incorporation. Here, we demonstrate an organic modifier incorporation strategy
capable of modulating the electronic density of states in halide tin
perovskites without altering the perovskite lattice, in a similar fashion to
substitutional doping in traditional semiconductors. By incorporating organic
small molecules and conjugated polymers into cesium tin iodide (CsSnI3)
perovskites, we achieve carrier density tunability over 2.7 decades, transition
from a semiconducting to a metallic nature, and high electrical conductivity
exceeding 200 S/cm. We leverage these tunable and enhanced electronic
properties to achieve a thin-film, lead free, thermoelectric material with a
near room-temperature figure-of-merit (ZT) of 0.21, the highest amongst all
halide perovskite thermoelectrics. Our strategy provides an additional degree
of freedom in the design of halide perovskites for optoelectronic and energy
applications.

###Theoretical insights on structural, electronic and thermoelectric properties of inorganic biphenylene: non-benzenoid Boron nitride|Ajay Kumar,Parbati Senapati,Prakash parida###

Theoretical insights on structural, electronic and thermoelectric properties of inorganic biphenylene: non-benzenoid Boron nitride. The first-principles calculations predict a stable biphenylene carbon network
(BPN) like the Boron-nitride structure named inorganic biphenylene network
(I-BPN). A comparison has been done between BPN and I-BPN to examine the
stability of the I-BPN monolayer. We calculate the formation energy, phonon
dispersion and mechanical parameters: young modulus and Poisson ratio for
mechanical stability. It has been found that the stability of I-BPN is
comparable with the BPN. The lattice transport properties reveal that the
phonon thermal conductivity of I-BPN is 10th order low than the BPN. The
electronic band structure reveals that I-BPN is a semiconductor with an
indirect bandgap of 1.88 eV with valence band maximum (VBM) at Y and conduction
band maximum (CBM) at the X high symmetry point. In addition, the
thermoelectric parameters, such as the seebeck coefficient, show the highest
peak value of 0.00292 V/K at 324K. Electronic transport properties reveal that
I-BPN is highly anisotropic along the x and y-axes. Furthermore, the
thermoelectric power factor as a function of chemical potential shows a peak
value of 0.0056 W/mK2 (900K) along the x-axis in the p-type doping region. An
electronic figure of merit shows an amplified peak approach to 1. The total
figure of merit (including lattice transport parameters) shows peak values of
0.378 (0.21) for p-type and 0.24 (0.198) n-type regions along the x(y)
direction. It is notice that the obtain ZT peaks values are higher than any B-N
compositions.

###Electron and phonon transport in shandite-structured Ni$_3$Sn$_2$S$_2$|Alex Aziz,Panagiotis Mangelis,Paz Vaqueiro,Anthony V. Powell,Ricardo Grau-Crespo###

Electron and phonon transport in shandite-structured Ni$_3$Sn$_2$S$_2$. The shandite family of solids, with hexagonal structure and composition
A3M2X2 (A = Ni,Co,Rh,Pd; M = Pb,In,Sn,Tl; X = S,Se), has attracted recent
research attention due to promising applications as thermoelectric materials.
Herein we discuss the electron and phonon transport properties of
shandite-structured Ni3Sn2S2, based on a combination of density functional
theory (DFT), Boltzmann transport theory, and experimental measurements.
Ni3Sn2S2 exhibits a metallic and non-magnetic groundstate with Ni$^0$ oxidation
state and very low charge on Sn and S atoms. Seebeck coefficients obtained from
theoretical calculations are in excellent agreement with those measured
experimentally between 100 and 600 K. From the calculation of the ratio
$\sigma$/$\tau$ between the electronic conductivity and relaxation time, and
the experimental determination of electron conductivity, we extract the
variation of the scattering rate (1/$\tau$) with temperature between 300 and
600 K, which turns out to be almost linear, thus implying that the dominant
electron scattering mechanism in this temperature range is via phonons. The
electronic thermal conductivity, which deviates only slightly from the
Wiedemann-Franz law, provides the main contribution to thermal transport. The
small lattice contribution to the thermal conductivity is calculated from the
phonon structure and third-order force constants, and is only ~2
Wm$^{-1}$K$^{-1}$ at 300 K (less than 10% of the total thermal conductivity),
which is confirmed by experimental measurements. Overall, Ni$_3$Sn$_2$S$_2$ is
a poor thermoelectric material (ZT~0.01 at 300 K), principally due to the low
absolute value of the Seebeck coefficient. However, the understanding of its
transport properties will be useful for the rationalization of the
thermoelectric behavior of other, more promising members of the shandite
family.

###Accurate high-throughput screening of I-II-V 8-electron Half-Heusler compounds for renewable-energy applications|Bhawna Sahni,Vikram,Jiban Kangsabanik,Aftab Alam###

Accurate high-throughput screening of I-II-V 8-electron Half-Heusler compounds for renewable-energy applications. Renewable energy resources have emerged as the best alternatives to fossil
fuel energy which are rapidly declining with time. Here, eight valence-electron
count Half-Heusler(HH) alloys have been studied using reliable first principles
calculations in the search of potential candidates for renewable energy
applications like thermoelectric (TE), solar harvesting, topological insulator
(TI) and transparent conductor (TC) applications. The initial screening
parameters used for our study are chemical and thermal stability, band gap,
nature of bandgap and band inversion strength. We have performed quasistatic
G0W0 calculation starting from HSE groundstate wavefunction to predict the most
accurate estimation of bandgap for these class of compounds. A total of 960
compounds were simulated. 121 out of 960 compounds were found to be thermally
and chemically stable. 31 compounds with bandgap less than 1.5 eV were studied
for thermoelectric application out of which 13 compounds were found to show
thermoelectric figure of merit ZT > 0.7 for both p-type and n-type conduction.
30 compounds with band gap 1-1.8 eV were studied for optoelectronic application
out of which 13 compounds were found to show Spectroscopic Limited Maximum
Efficiency (SLME) more than 20%, comparable to existing state of the art
materials. 21 compounds were found to show band inversion at ambient conditions
which is a necessary condition for topological insulators. The surface band
structure calculations for one of the promising candidate was done to check
robustness of the topological behaviour. 29 compounds were found to have
bandgap more than 2 eV which are promoted for transparent conductor
applications with further band engineering. We strongly believe that our
calculations will give useful insights to experimentalists for synthesizing and
investigating proposed compounds for different energy applications.

###First-principles calculations to investigate structural, elastic, electronic, thermodynamic, and thermoelectric properties of CaPd$_3$B$_4$O$_{12}$ (B = Ti, V) perovskite|M. H. K. Rubel,M. A. Hossain,M. Khalid Hossain,K. M. Hossain,A. A. Khatun,M. M. Rahaman,Md. Ferdous Rahman,M. M. Hossain,J. Hossain###

First-principles calculations to investigate structural, elastic, electronic, thermodynamic, and thermoelectric properties of CaPd$_3$B$_4$O$_{12}$ (B = Ti, V) perovskite. This study has explored numerous physical properties of
CaPd$_3$Ti$_4$O$_{1}$2 (CPTO) and CaPd$_3$V$_4$O$_{12}$ (CPVO) quadruple
perovskites employing the density functional theory (DFT) method. The
mechanical permanence of these two compounds was observed by the Born stability
criteria as well. The band structure of CPTO reveals a 0.88 and 0.46 eV direct
narrow band gap while using GGA-mBJ and GGA-PBE potentials, respectively, which
is an indication of its fascinating semiconducting nature. The calculated
partial density of states indicates the strong hybridization between Pd-4d and
O-2p orbital electrons for CPTO, whereas Pd-4d and V-3d-O-2p for CPVO. The
study of the chemical bonding nature and electronic charge distribution graph
reveals the coexistence of covalent O-V/Pd bonds, ionic O-Ti/Ca bonds, as well
as metallic Ti/V-Ti/V bonding for both compounds. The Fermi surface of CPVO
ensures a kind of hole as well as electron faces simultaneously, indicating the
multifarious band characteristic. The prediction of the static real dielectric
function (optical property) of CPTO at zero energy implies its promising
dielectric nature. The photoconductivity and absorption coefficient of CPBO
display good qualitative compliance with the consequences of band structure
computations. The calculated thermodynamic properties manifest the
thermodynamical stability for CPBO, whereas phonon dispersions of CPVO exhibit
stable phonon dispersion in contrast to slightly unstable phonon dispersion of
CPTO. The predicted Debye temperature ($\theta_D$) has been utilized to
correlate its topical features including thermoelectric behaviors. The studied
thermoelectric transport properties of CPTO yielded the Seebeck coefficient
(186 V/K), power factor (11.9 Wcm$^{-1}$K$^{-2}$), and figure of merit (ZT)
value of about 0.8 at 800 K, indicating that this material could be a promising
candidate for thermoelectric applications.

###Thermoelectric properties, efficiency and thermal expansion of ZrNiSn half-Heusler by first-principles calculations|Shivprasad S. Shastri,Sudhir K. Pandey###

Thermoelectric properties, efficiency and thermal expansion of ZrNiSn half-Heusler by first-principles calculations. In this work, we try to understand the experimental thermoelectric (TE)
properties of a ZrNiSn sample with DFT and semiclassical transport calculations
using SCAN functional. SCAN and mBJ provide the same band gap $E_{g}$ of
$\sim$0.54 eV. This $E_{g}$ is found to be inadequate to explain the
experimental data. The better explanation of experimental Seebeck coefficient
$S$ is done by considering $E_{g}$ of 0.18 eV which suggests the
non-stoichiometry and/or disorder in the sample. Further improvement in the $S$
is done by the inclusion of temperature dependence on chemical potential. In
order to look for the possible enhanced TE properties obtainable in ZrNiSn with
$E_{g}$ of $\sim$0.54 eV, power factor and optimal carrier concentrations are
calculated. The optimal electron and hole concentrations required to attain
highest power factors are $\sim$7.6x10$^{19}$ cm$^{-3}$ and $\sim$1.5x10$^{21}$
cm$^{-3}$, respectively. The maximum figure of merit $ZT$ calculated at 1200 K
for n-type and p-type ZrNiSn are $\sim$0.6 and $\sim$0.7, respectively. The %
efficiency obtained for n-type ZrNiSn is $\sim$5.1 % while for p-type ZrNiSn is
$\sim$6.1 %. The $ZT$ are expected to be further enhanced to $\sim$1.2 (n-type)
and $\sim$1.4 (p-type) at 1200 K by doping with heavy elements for thermal
conductivity reduction. The phonon properties are also studied by calculating
dispersion, total and partial density of states. The calculated Debye
temperature of 382 K is in good agreement with experimental value of 398 K. The
thermal expansion behaviour in ZrNiSn is studied under quasi-harmonic
approximation. The average linear thermal expansion coefficient
$\alpha_{ave}(T)$ of $\sim$7.8x10$^{-6}$ K$^{-1}$ calculated in our work is
quite close to the experimental values.

###Thermal spin transport and spin in thermoelectrics|Joseph P. Heremans###

Thermal spin transport and spin in thermoelectrics. This article reviews the principles that govern the combined transport of
spin, heat, and charge. The extensive thermodynamic quantity associated with
spin transport is the magnetization; its Onsager-conjugate force is in general
the derivative of the free energy with respect to the magnetization. Spins are
carried in one of two ways: (1) by spin-polarized free electrons in magnetic
metals and doped semiconductors, or (2) by spin waves (magnons) that reside on
localized electrons on unfilled d- or f-shells of transition metal or
rare-earth elements. The paper covers both cases in separate chapters. In both
cases, it is possible to define a spin chemical potential whose gradient is the
more practical conjugate force to spin transport. The paper further describes
the anomalous Hall, spin Hall, and inverse spin Hall effects in magnetic and
non-magnetic solids with strong spin-orbit coupling because these effects are
used to generate and measure spin fluxes. Spin transport across interfaces is
described next, and includes spin pumping and spin transfer torque. The final
chapter then puts all these concepts together to describe the spin-Seebeck,
spin-Peltier, and magnon-drag effects, which exist in ferromagnetic,
antiferromagnetic, and even paramagnetic solids. Magnon-drag, in particular, is
a high-temperature effect that boosts the thermopower of metals by an order of
magnitude and that of semiconductors by a factor of 2 or 3 above the electronic
diffusion thermopower. This is the only example where a spin-driven effect is
larger than a charge-driven effect. Magnon drag leads a simple binary
paramagnetic semiconductor, MnTe, to have zT > 1 without optimization. This
shows how adding spin as an additional design parameter in thermoelectrics
research is a new and promising approach toward the quest for high-zT
materials.

###Thermodynamics of the mono-energetic energy selective Contacts of the hot carrier solar cell|Antonio Martí,Elisa Antolín,Iñigo Ramiro###

Thermodynamics of the mono-energetic energy selective Contacts of the hot carrier solar cell. The hot carrier solar cell (HCSC) has the potential for converting solar
energy into electrochemical energy with an efficiency of 85.4%. For this, in
addition to an idealized light absorber, the HCSC has to be connected to the
external load by means of the so-called \emph{mono-energetic energy selective
contacts} (ESCs). However, the thermodynamic properties that these types of
contact have to exhibit, such as their electric, thermal conductivity and
Seebeck coefficient, have not been explored. This paper aims to fill this gap.
In this respect, we model electron transport in non-ideal ESCs using the
transport theory proposed by Datta and Landauer which has allowed us to
calculate the value of these parameters as a function of the temperature and
electrochemical potential of operation. Our findings also reveal that, to
preserve the HCSC efficiency above 82%, the ESCs could require in the order of
$3 \times 10^{19}$ cm$^{-3}$ electron states. As the ESCs depart from ideality,
the temperature of the hot carriers at which optimum efficiency is obtained
increases to above 2540 K. The mono-enenergetic selective contact characterized
by the highest energy demands an electric, thermal conductivity and Seebeck
coefficient that, when combined, are characterized by a high thermoelectric
figure of merit $(ZT\approx 8)$. We are not aware of any material exhibiting
this figure of merit which illustrates the difficulty in putting the HCSC
concept into practice. Conversely, our work supports the idea that pursuing
materials capable of transporting electrons ballistically through
mono-energetic electron channels can provide the key for achieving materials
characterized by high $ZT$

###Vibrational cooling and thermoelectric response of nanoelectromechanical systems|Liliana Arrachea,Niels Bode,Felix von Oppen###

Vibrational cooling and thermoelectric response of nanoelectromechanical systems. An important goal in nanoelectromechanics is to cool the vibrational motion,
ideally to its quantum ground state. Cooling by an applied charge current is a
particularly simple and hence attractive strategy to this effect. Here, we
explore this phenomenon in the context of the general theory of
thermoelectrics. In linear response, this theory describes thermoelectric
refrigerators in terms of their cooling efficiency and figure of merit ZT. We
show that both concepts carry over to phonon cooling in nanoelectromechanical
systems. As an important consequence, this allows us to discuss the efficiency
of phonon refrigerators in relation to the fundamental Carnot efficiency. We
illustrate these general concepts by thoroughly investigating a simple
double-quantum-dot model with the dual advantage of being quite realistic
experimentally and amenable to a largely analytical analysis theoretically.
Specifically, we obtain results for the efficiency, the figure of merit, and
the effective temperature of the vibrational motion in two regimes. In the
quantum regime in which the vibrational motion is fast compared to the
electronic degrees of freedom, we can describe the electronic and phononic
dynamics of the model in terms of master equations. In the complementary
classical regime of slow vibrational motion, the dynamics is described in terms
of an appropriate Langevin equation. Remarkably, we find that the efficiency
can approach the maximal Carnot value in the quantum regime, with large
associated figures of merit. In contrast, the efficiencies are typically far
from the Carnot limit in the classical regime. Our theoretical results should
provide guidance to implementing efficient vibrational cooling of
nanoelectromechanical systems in the laboratory.

###Thermoelectric Alchemy: Designing A Chemical Analog to PbTe with Intrinsic High Band Degeneracy and Low Lattice Thermal Conductivity|Jiangang He,Yi Xia,S. Shahab Naghavi,Vidvuds Ozoliņš,Chris Wolverton###

Thermoelectric Alchemy: Designing A Chemical Analog to PbTe with Intrinsic High Band Degeneracy and Low Lattice Thermal Conductivity. Improving the figure of merit $zT$ of thermoelectric materials requires
simultaneously a high power factor and low thermal conductivity. An effective
approach for increasing the power factor is to align the band extremum and
achieve high band degeneracy ($\geq$ 12) near the Fermi level as realized in
PbTe [\textcolor{blue}{Pei et. al. \textit{Nature} 473, 66 (2010)}], which
usually relies on band structure engineering, e.g., chemical doping and strain.
However, very few materials could achieve such a high band degeneracy without
heavy doping or suffering impractical strain. By employing state-of-the-art
first-principles methods with direct computation of phonon and carrier
lifetime, we demonstrate that two new full-Heusler compounds Li$_2$TlBi and
Li$_2$InBi, possessing a PbTe-like electronic structure, show exceptionally
high power factors ($\sim$ 20 mWm$^{-1}$K$^{-2}$ at 300 K) and low lattice
thermal conductivities (2.36 and 1.55 Wm$^{-1}$K$^{-1}$) at room temperature.
The Tl$^{+}$Bi$^{3-}$ (In$^{+}$Bi$^{3-}$) sublattice forms a rock-salt
structure, and the additional two valence electrons from Li atoms essentially
make these compounds isovalent with Pb$^{2+}$Te$^{2-}$. The larger rock-salt
sublattice of TlBi (InBi) shifts the valence band maximum from L point to the
middle of the $\Sigma$ line, increasing the band degeneracy from fourfold to
twelvefold. On the other hand, resonance bond in the PbTe-like sublattice and
soft Tl-Bi (In-Bi) bonding interaction is responsible for intrinsic low lattice
thermal conductivities. Our results present a novel strategy of designing
high-performance thermoelectric materials.

###First-principles study of optoelectronic and thermoelectronic properties of the ScAgC half-Heusler compound|Vinod Kumar Solet,Shamim Sk,Sudhir K. Pandey###

First-principles study of optoelectronic and thermoelectronic properties of the ScAgC half-Heusler compound. This work presents a theoretical study regarding photovoltaic (PV) &
thermoelectric (TE) applications of ScAgC. The electronic, optical and
thermoelectric properties are investigated using DFT and semi-classical
Boltzmann transport theory. DFT calculates a direct band-gap of $\sim$$0.47$
eV, while $G_{0}W_{0}$ method estimates a band-gap of $\sim1.01$ eV. We used
parabola fitting to estimate effective mass ($m^{*}$) values for bands B1-B4 at
$\Gamma$-point, which are $\sim$ -0.087 (-0.075), $\sim$ -0.17 (-0.27), $\sim$
-0.17 (-0.27), and $\sim$ 0.049 (0.058) along the $\Gamma-X$ ($\Gamma-L$)
direction, respectively. We also examine phonon dispersion and thermal
properties. Furthermore, optoelectronics' properties are calculated within the
energy range of 0 to 10 eV. The optical conductivity $\sigma (\omega)$,
refractive index $\tilde{n}(\omega)$, and dielectric function $\epsilon
(\omega)$ show strong optical transitions in the visible region. The lowest
calculated value of reflectivity r($\omega$) is $\sim$0.24 at $\sim$4.7 eV, and
the highest calculated value of absorption coefficient $\alpha (\omega )$ is
$\sim1.7\times 10^{6} cm^{-1}$ at $\sim$ 8.5 eV. At 300 K, we expect a maximum
solar efficiency (SLME) of $\sim$33% at $\sim$1 $\mu m$. The lattice thermal
conductivity $\kappa_{ph}$ shows maximum value of $\sim$3.8 W$m^{-1}K^{-1}$ at
1200 K. At 1200 K, for electron doping of $\sim$3.9$\times$$10^{21}$$cm^{-3}$,
the maximum value of $S^{2}\sigma /\tau$ is $\sim$145 $\times 10^{14}$ $\mu
WK^{-2}cm^{-1}s^{-1}$, while for hole doping of
$\sim$1.5$\times$$10^{21}$$cm^{-3}$, it is $\sim$123 $\times 10^{14}$ $\mu
WK^{-2}cm^{-1}s^{-1}$. The highest $ZT$ at 1200 K is expected to be $\sim$0.53,
whereas the optimal \%efficiency is predicted as $\sim$8.5% for cold (hot)
temperatures of 300 (1200) K. The results suggest that ScAgC can be a potential
candidate for solar cell and TE applications.

###Transport properties of Layer-Antiferromagnet CuCrS2: A possible thermoelectric material|G. C. Tewari,T. S. Tripathi,A. K. Rastogi###

Transport properties of Layer-Antiferromagnet CuCrS2: A possible thermoelectric material. The electrical, thermal conductivity and Seebeck coefficient of the quenched,
annealed and slowly cooled phases of the layer compound CuCrS2 have been
reported between 15K to 300K. We also confirm the antiferromagnetic transition
at 40K in them by our magnetic measurements between 2K and 300K. The crystal
flakes show a minimum around 100K in their in-plane resistance behavior. For
the polycrystalline pellets the resistivity depends on their flaky texture and
it attains at most 10 to 20 times of the room temperature value at the lowest
temperature of measurement. The temperature dependence is complex and no
definite activation energy of electronic conduction can be discerned. We find
that the Seebeck coefficient is between 200-450 microV/K and is unusually large
for the observed resistivity values of between 5-100 mOhm-cm at room
temperature. The figure of merit ZT for the thermoelectric application is 2.3
for our quenched phases, which is much larger than 1 for useful materials. The
thermal conductivity K is mostly due to lattice conduction and is reduced by
the disorder in Cu- occupancy in our quenched phase. A dramatic reduction of
electrical and thermal conductivity is found as the antiferromagnetic
transition is approached from the paramagnetic region, and K subsequently rises
in the ordered phase. We discuss the transport properties as being similar to a
doped Kondo-insulator.

###Large enhancement of the thermoelectric figure of merit in a ridged quantum well|Avto Tavkhelidze###

Large enhancement of the thermoelectric figure of merit in a ridged quantum well. Recently new quantum features were observed and studied in the ridged quantum
wells (RQW). Periodic ridges on the surface of the quantum well layer impose
additional boundary conditions on electron wave function and reduce quantum
state density. As result, chemical potential of RQW increases and becomes the
ridge height dependent. Here we propose system comprising of RQW and additional
layer on the top of the ridges forming periodic series of p+-n+ junctions (or
metal-n+ junctions). In such systems charge depletion region develops inside
the ridges and effective ridge height reduces, becoming rather strong function
of temperature T. Consequently, T dependence of chemical potential magnifies
and Seebeck coefficient S increases. We investigate S in the system of
semiconductor RQW having abrupt p+-n+ junctions or metal-n+ junctions on the
top of the ridges. Analysis made on the basis of Boltzmann transport equations
shows dramatic increase in S for both cases. At the same time other transport
coefficients remain unaffected by the junctions. Calculations show one order of
magnitude increase in thermoelectric figure of merit ZT relative to the bulk
material.

###Hinge-like structure induced unusual properties of black phosphorus and new strategies to improve the thermoelectric performance|Guangzhao Qin,Qing-Bo Yan,Zhenzhen Qin,Sheng-Ying Yue,Hui-Juan Cui,Qing-Rong Zheng,Gang Su###

Hinge-like structure induced unusual properties of black phosphorus and new strategies to improve the thermoelectric performance. We systematically investigated the geometric, electronic and thermoelectric
(TE) properties of bulk black phosphorus (BP) under strain. The hinge-like
structure of BP brings unusual mechanical responses such as anisotropic Young's
modulus and negative Poisson's ratio. A sensitive electronic structure of BP
makes it transform among metal, direct and indirect semiconductors under
strain. The maximal figure of merit $ZT$ of BP is found to be 0.72 at
$800\,\mathrm{K}$ that could be enhanced to 0.87 by exerting an appropriate
strain, revealing BP could be a potential medium-high temperature TE material.
Such strain-induced enhancements of TE performance are often observed to occur
at the boundary of the direct-indirect band gap transition, which can be
attributed to the increase of degeneracy of energy valleys at the transition
point. By comparing the structure of BP with SnSe, a family of potential TE
materials with hinge-like structure are suggested. This study not only exposes
various novel properties of BP under strain, but also proposes effective
strategies to seek for better TE materials.

###Atomistic calculation of the thermoelectric properties of Si nanowires|Igor Bejenari,Peter Kratzer###

Atomistic calculation of the thermoelectric properties of Si nanowires. The thermoelectric properties of 1.6 nm-thick Si square nanowires with [100]
crystalline orientation are calculated over a wide temperature range from 0 K
to 1000 K, taking into account atomistic electron-phonon interaction. In our
model, the [010] and [001] facets are passivated by hydrogen and there are
Si-Si dimers on the nanowire surface. The electronic structure was calculated
by using the sp^3 spin-orbit-coupled atomistic second-nearest-neighbor
tight-binding model. The phonon dispersion was calculated from a valence force
field model of the Brenner type. A scheme for calculating electron-phonon
matrix elements from a second-nearest neighbor tight-binding model is
presented. Based on Fermi's golden rule, the electron-phonon transition rate
was obtained by combining the electron and phonon eigenstates. Both elastic and
inelastic scattering processes are taken into consideration. The temperature
dependence of transport characteristics was calculated by using a solution of
linearized Boltzmann transport equation obtained by means of the iterative
Orthomin method. At room temperature, the electron mobility is 195 cm^2/(Vs)
and increases with temperature, while a figure-of-mertit ZT=0.38 is reached for
n-type doping with a concentration of n=10^19 cm^-3.

###Thermoelectric power factor of a 70 nm Ni-nanowire in a magnetic field|Rüdiger Mitdank,Martin Handwerg,Corinna Steinweg,William Töllner,Mihaela Daub,Kornelius Nielsch,Saskia F. Fischer###

Thermoelectric power factor of a 70 nm Ni-nanowire in a magnetic field. Thermoelectric (TE) properties of a single nanowire (NW) are investigated in
a microlab which allows the determination of the Seebeck coefficient S and the
conductivity {\sigma}. A significiant influence of the magnetization of a 70 nm
ferromagnetic Ni-NW on its power factor S^{2}{\sigma} is observed. We detected
a strong magneto thermopower effect (MTP) of about 10% and an anisotropic
magneto resistance (AMR) as a function of an external magnetic field B in the
order of 1%. At T = 295 K and B = 0 T we determined the absolute value of S = -
(19 \pm 2) {\mu}V/K. At zero field the figure of merit ZT \approx 0.02 was
calculated using the Wiedemann-Franz-law for the thermal conductivity. The
thermopower S increases considerably as a function of B up to 10% at B = 0.5 T,
and with a magneto thermopower of \partialS/\partialB \approx - (3.8 \pm 0,5)
{\mu}V/(K.T). The AMR and MTP are related by \partials/\partialr \approx -11
\pm 1 (\partials = \partialS/S). The TE efficiency increases in a transversal
magnetic field (B =0.5T) due to an enhanced power factor by nearly 20%.

###Phosphorene nanoribbon as a promising candidate for thermoelectric applications|J. Zhang,H. J. Liu,L. Cheng,J. Wei,J. H. Liang,D. D. Fan,J. Shi,X. F. Tang,Q. J. Zhang###

Phosphorene nanoribbon as a promising candidate for thermoelectric applications. In this work, the electronic properties of phosphorene nanoribbons with
different width and edge configurations are studied by using density functional
theory. It is found that the armchair phosphorene nanoribbons are
semiconducting while the zigzag nanoribbons are metallic. The band gaps of
armchair nanoribbons decrease monotonically with increasing ribbon width. By
passivating the edge phosphorus atoms with hydrogen, the zigzag series also
become semiconducting, while the armchair series exhibit a larger band gap than
their pristine counterpart. The electronic transport properties of these
phosphorene nanoribbons are then investigated using Boltzmann theory and
relaxation time approximation. We find that all the semiconducting nanoribbons
exhibit very large values of Seebeck coefficient and can be further enhanced by
hydrogen passivation at the edge. Taking armchair nanoribbon with width N=7 as
an example, we calculate the lattice thermal conductivity with the help of
phonon Boltzmann transport equation. Due to significantly enhanced Seebeck
coefficient and decreased thermal conductivity, the phosphorene nanoribbon
exhibit a very high figure of merit (ZT value) of 4.0 at room temperature,
which suggests its appealing thermoelectric applications.

###Ferromagnetically correlated clusters in semi-metallic Ru2NbAl Heusler alloy|Sanchayita Mondal,Chandan Mazumdar,R. Ranganathan,Eric Alleno,P. C. Sreeparvathy,V. Kanchana,G. Vaitheeswaran###

Ferromagnetically correlated clusters in semi-metallic Ru2NbAl Heusler alloy. In this work, we report the structural, magnetic and electrical and thermal
transport properties of the Heusler-type alloy Ru2NbAl. From the detailed
analysis of magnetization data, we infer the presence of superparamagnetically
interacting clusters with a Pauli paramagnetic background, while short-range
ferromagnetic interaction is developed among the clusters below 5 K. The
presence of this ferromagnetic interaction is confirmed through heat capacity
measurements. The relatively small value of electronic contribution to specific
heat, gamma (~2.7 mJ/mol-K2), as well as the linear nature of temperature
dependence of Seebeck coefficient indicate a semi-metallic ground state with a
pseudo-gap that is also supported by our electronic structure calculations. The
activated nature of resistivity is reflected in the observed negative
temperature coefficient and has its origin in the charge carrier localization
due to antisite defects, inferred from magnetic measurements as well as
structural analysis. Although the absolute value of thermoelectric figure of
merit is rather low (ZT = 5.2*10-3) in Ru2NbAl, it is the largest among all the
reported non-doped full Heusler alloys.

###New two-dimensional phase of tin chalcogenides: candidates for high-performance thermoelectric materials|Baojuan Dong,Zhenhai Wang,Nguyen T. Hung,Artem R. Oganov,Teng Yang,Riichiro Saito,Zhidong Zhang###

New two-dimensional phase of tin chalcogenides: candidates for high-performance thermoelectric materials. Tin-chalcogenides SnX (X = Te, Se and S) have been arousing research interest
due to their thermoelectric physical properties. The two-dimensional (2D)
counterparts, which are expected to enhance the property, nevertheless, have
not been fully explored because of many possible structures. Generating
variable composition of 2D Sn$_{1-x}$X$_{x}$ systems (X = Te, Se and S) has
been performed using global searching method based on evolutionary algorithm
combining with density functional calculations. A new hexagonal phase named by
$\beta'$-SnX is found by Universal Structure Predictor Evolutionary
Xtallography (USPEX), and the structural stability has been further checked by
phonon dispersion calculation and the elasticity criteria. The $\beta'$-SnTe is
the most stable among all possible 2D phases of SnTe including those
experimentally available phases. Further, $\beta'$ phases of SnSe and SnS are
also found energetically close to the most stable phases. High thermoelectronic
(TE) performance has been achieved in the $\beta'$-SnX phases, which have
dimensionless figure of merit (ZT) as high as $\sim$0.96 to 3.81 for SnTe,
$\sim$0.93 to 2.51 for SnSe and $\sim$1.19 to 3.18 for SnS at temperature
ranging from 300 K to 900 K with practically attainable carrier concentration
of 5$\times$10$^{12}$ cm$^{-2}$. The high TE performance is resulted from a
high power factor which is attributed to the quantum confinement of 2D
materials and the band convergence near Fermi level, as well as low thermal
conductivity mainly from both low elastic constants due to weak inter-Sn
bonding strength and strong lattice anharmonicity.

###Separation of heat and charge currents for boosted thermoelectric conversion|Francesco Mazza,Stefano Valentini,Riccardo Bosisio,Giuliano Benenti,Vittorio Giovannetti,Rosario Fazio,Fabio Taddei###

Separation of heat and charge currents for boosted thermoelectric conversion. In a multi-terminal device the (electronic) heat and charge currents can
follow different paths. In this paper we introduce and analyse a class of
multi-terminal devices where this property is pushed to its extreme limits,
with charge $and$ heat currents flowing in different reservoirs. After
introducing the main characteristics of such $heat-charge$ $current$
$separation$ regime we show how to realise it in a multi-terminal device with
normal and superconducting leads. We demonstrate that this regime allows to
control independently heat and charge flows and to greatly enhance
thermoelectric performances at low temperatures. We analyse in details a
three-terminal setup involving a superconducting lead, a normal lead and a
voltage probe. For a generic scattering region we show that in the regime of
heat-charge current separation both the power factor and the figure of merit
$ZT$ are highly increased with respect to a standard two-terminal system. These
results are confirmed for the specific case of a system consisting of three
coupled quantum dots.

###Critical mode and band-gap-controlled bipolar thermoelectric properties of SnSe|I. Loa,S. R. Popuri,A. D. Fortes,J. W. G. Bos###

Critical mode and band-gap-controlled bipolar thermoelectric properties of SnSe. The reliable calculation of electronic structures and understanding of
electrical properties depends on an accurate model of the crystal structure.
Here, we have reinvestigated the crystal structure of the high-zT
thermoelectric material tin selenide, SnSe, between 4 and 1000 K using
high-resolution neutron powder diffraction. Symmetry analysis reveals the
presence of four active structural distortion modes, one of which is found to
be active over a relatively wide range of more than +/-200 K around the
symmetry-breaking Pnma-Cmcm transition at 800 K. Density functional theory
calculations on the basis of the experimental structure parameters show that
the unusual, step-like temperature dependencies of the electrical transport
properties of SnSe are caused by the onset of intrinsic bipolar conductivity,
amplified and shifted to lower temperatures by a rapid reduction of the band
gap between 700 and 800 K. The calculated band gap is highly sensitive to small
out-of-plane Sn displacements observed in the diffraction experiments. SnSe
with a sufficiently controlled acceptor concentration is predicted to produce
simultaneously a large positive and a large negative Seebeck effect along
different crystal directions.

###The use of strain and grain boundaries to tailor phonon transport properties: A first principles study of 2H-phase $CuAlO_{2}$ (Part II)|Evan Witkoske,Zhen Tong,Yining Feng,Xiulin Ruan,Mark Lundstrom,Na Lu###

The use of strain and grain boundaries to tailor phonon transport properties: A first principles study of 2H-phase $CuAlO_{2}$ (Part II). Transparent oxide materials, such as $CuAlO_{2}$, a p-type transparent
conducting oxide (TCO), have recently been studied for high temperature
thermoelectric power generators and coolers for waste heat. TCO materials are
generally low cost and non-toxic. The potential to engineer them through strain
and nano-structuring are two promising avenues toward continuously tuning the
electronic and thermal properties to achieve high zT values and low cost/kW-hr
devices. In this work, the strain-dependent lattice thermal conductivity of 2H
$CuAlO_{2}$ is computed by solving the phonon Boltzmann transport equation with
interatomic force constants extracted from first-principles calculations. While
the average bulk thermal conductivity is around 32 W/(K-m) at room temperature,
it drops to between 5-15 W/(K-m) for typical experimental grain sizes from 3nm
to 30nm at room temperature. We find that strain can offer both an increase as
well as a decrease in the thermal conductivity as expected, however the overall
inclusion of small grain sizes dictates the potential for low thermal
conductivity in this material.

###Thermoelectric transport properties in graphene connected molecular junctions|S. T. Rodriguez,I. Grosu,M. Crisan,I. Tifrea###

Thermoelectric transport properties in graphene connected molecular junctions. We study the electronic contribution to the main thermoelectric properties of
a molecular junction consisting of a single quantum dot coupled to graphene
external leads. The system electrical conductivity (G), Seebeck coefficient
($S$), and the thermal conductivity ($\kappa$), are numerically calculated
based on a Green's function formalism that includes contributions up to the
Hartree-Fock level. We consider the system leads to be made either of pure or
gapped-graphene. To describe the free electrons in the gapped-graphene
electrodes we used two possible scenarios, the massive gap scenario, and the
massless gap scenario, respectively. In all cases, the Fano effect is
responsible for a strong violation of the Wiedemann-Franz law and we found a
substantial increase of the system figure of merit $ZT$ due to a drastic
reduction of the system thermal coefficient. In the case of gapped-graphene
electrodes, the system figure of merit presents a maximum at an optimal value
of the energy gap of the order of $\Delta/D\sim$ 0.002 (massive gap scenario)
and $\Delta/D\sim$ 0.0026 (massless gap scenario). Additionally, for all cases,
the system figure of merit is temperature dependent.

###A sharp increase in the density of states in PbTe approaching a saddle point in the band structure|P. Walmsley,D. M. Abrams,J. Straquadine,M. K. Chan,R. D. McDonald,P. Giraldo-Gallo,I. R. Fisher###

A sharp increase in the density of states in PbTe approaching a saddle point in the band structure. PbTe is a leading mid-range thermoelectric material with a $zT$ that has been
enhanced by, amongst other methods, band engineering. Here we present an
experimental study of the Hall effect, quantum oscillations, specific heat, and
electron microprobe analysis that explores the evolution of the electronic
structure of PbTe heavily doped with the `ideal' acceptor Na up to the
solubility limit. We identify two phenomenological changes that onset as the
electronic structure deviates from a Kane-type dispersion at around 180meV; a
qualitative change in the field dependence of the Hall effect indicative of an
increase in the high-field limit and a change in the Fermiology, and a sharp
increase in the density of states as a function of energy. Following
consideration of three possible origins for the observed phenomenology we
conclude that the most likely source is non-ellipsoidicity of the $L$-pocket
upon approach to a saddle point in the band structure, which is evidenced
directly by our quantum oscillation measurements. Comparison to density
functional theory calculations imply that this evolution of the electronic
structure may be a key contributor to the large thermopower in PbTe.

###Chemical Aspects of the Antiferromagnetic Topological Insulator MnBi$_{2}$Te$_{4}$|Alexander Zeugner,Frederik Nietschke,Anja U. B. Wolter,Sebastian Gaß,Raphael C. Vidal,Thiago R. F. Peixoto,Darius Pohl,Christine Damm,Axel Lubk,Richard Hentrich,Simon K. Moser,Celso Fornari,Chul Hee Min,Sonja Schatz,Katharina Kißner,Maximilian Ünzelmann,Martin Kaiser,Francesco Scaravaggi,Bernd Rellinghaus,Kornelius Nielsch,Christian Heß,Bernd Büchner,Friedrich Reinert,Hendrik Bentmann,Oliver Oeckler,Thomas Doert,Michael Ruck,Anna Isaeva###

Chemical Aspects of the Antiferromagnetic Topological Insulator MnBi$_{2}$Te$_{4}$. Crystal growth of MnBi$_{2}$Te$_{4}$ has delivered the first experimental
corroboration of the 3D antiferromagnetic topological insulator state. Our
present results confirm that the synthesis of MnBi$_{2}$Te$_{4}$ can be
scaled-up and strengthen it as a promising experimental platform for studies of
a crossover between magnetic ordering and non-trivial topology. High-quality
single crystals of MnBi$_{2}$Te$_{4}$ are grown by slow cooling within a narrow
range between the melting points of Bi$_{2}$Te$_{3}$ (586 {\deg}C) and
MnBi$_{2}$Te$_{4}$ (600 {\deg}C). Single crystal X-ray diffraction and electron
microscopy reveal ubiquitous antisite defects in both cation sites and,
possibly, Mn vacancies. Powders of MnBi$_{2}$Te$_{4}$ can be obtained at
subsolidus temperatures, and a complementary thermochemical study establishes a
limited high-temperature range of phase stability. Nevertheless, quenched
powders are stable at room temperature and exhibit long-range antiferromagnetic
ordering below 24 K. The expected Mn(II) out-of-plane magnetic state is
confirmed by the magnetization, X-ray photoemission, X-ray absorption and
linear dichroism data. MnBi$_{2}$Te$_{4}$ exhibits a metallic type of
resistivity in the range 4.5-300 K. The compound is an n-type conductor that
reaches a thermoelectric figure of merit up to ZT = 0.17. Angle-resolved
photoemission experiments provide evidence for a surface state forming a gapped
Dirac cone.

###Efficient and tunable Aharonov-Bohm quantum heat engine|Géraldine Haack,Francesco Giazotto###

Efficient and tunable Aharonov-Bohm quantum heat engine. We propose a quantum heat engine based on an Aharonov-Bohm interferometer in
a two-terminal geometry, and investigate its thermoelectric performances in the
linear response regime. Sizeable thermopower (up to $\sim 0.3\,\text{mV}$/K) as
well as $ZT$ values largely exceeding unity can be achieved by simply adjusting
parameters of the setup and temperature bias across the interferometer leading
to thermal efficiency at maximum power approaching $30\%$ of the Carnot limit.
This is close to the optimal efficiency at maximum power achievable for a
two-terminal heat engine. Changing the magnetic flux, the asymmetry of the
structure, a side-gate bias voltage through a capacitively-coupled electrode
and the transmission of the T-junctions connecting the AB ring to the contacts
allows to finely tune the operation of the quantum heat engine. The exploration
of the parameters' space demonstrates that the high performances of the
Aharonov-Bohm two-terminal device as a quantum heat engine are stable over a
wide range of temperatures and length imbalances, promising towards
experimental realization.

###Ultralow lattice thermal conductivity and electronic properties of monolayer 1T phase semimetal SiTe2 and SnTe2|Yi Wang,Zhibin Gao,Jun Zhou###

Ultralow lattice thermal conductivity and electronic properties of monolayer 1T phase semimetal SiTe2 and SnTe2. 2H phase (trigonal prismatic D3h) of layered two-dimensional (2D) transition
metal dichalcogenides (TMDs) have attracted a lot of interests due to the
superior electronic and optoelectronic properties. However, flexible electronic
devices and thermoelectric performances based on 2H phase have been potentially
limited by the strain sensitive electronic band gap and high lattice thermal
conductivity (kappa_L). Here, we predict and calculate two 1T (octahedral Oh)
phase monolayer telluride materials SnTe2 and SiTe2 with soft mechanics,
ultralow kappa_L and electronic properties. The calculated in-plane Young's
modulus of monolayer SnTe2 is softer than most of 1T-MX2 compounds.
Furthermore, monolayer SiTe2 and SnTe2 also have relatively flexible electronic
properties under large biaxial strain, indicating potential flexible electrode
materials. Meanwhile, monolayer SiTe2 and SnTe2 both exhibit ultralow
\k{appa}_L (2.27 W/mK of SiTe2 and 1.62 W/mK of SnTe2) at room temperature.
Considering both acoustic and polar optical phonon scattering of the electronic
relaxation time, the figure of merit (ZT) can achieve 0.46 at 600 K and 0.71 at
900 K for monolayer SiTe2 and SnTe2 respectively.

###Magnetic Constitution of Topologically Trivial Thermoelectric PbTe:Cr|Katarzyna Gas,Aleksandra Krolicka,Krzysztof Dybko,Piotr Nowicki,Zeinab Khosravizadeh,Tomasz Story,Maciej Sawicki###

Magnetic Constitution of Topologically Trivial Thermoelectric PbTe:Cr. In this paper we report on detailed temperature and magnetic field dependence
of m agnetization of IV-VI semiconductor PbTe doped with mixed valence
transition metal Cr$^{2+/3+}$. The material is studied solely by an integral
superconducting quantum interference device magnetometer in order to
quantitatively determine the contribution of single substitutional Cr$^{3+}$ as
well as of various Cr-Te magnetic nanocrystals, including their identification.
The applied experimental procedure reveals the presence of about
$10^{19}$~cm$^{-3}$ paramagnetic Cr$^{3+}$ ions formed via self-ionization of
Cr$^{2+}$ resonant donors. These are known to improve the thermoelectric figure
of merit parameter zT of this semiconductor. The magnetic finding excellently
agrees with previous Hall effect studies thus providing a new experimental
support for the proposed electronic structure model of PbTe:Cr system with
resonant Cr$^{2+/3+}$ state located (at low temperatures) about 100 meV above
the bottom of the conduction band. Below room temperature a ferromagnetic-like
signal points to the presence of Cr-rich nanocrystalline precipitates. Two most
likely candidates, namely: Cr$_2$Te$_3$ and Cr$_5$Te$_8$ are identified upon
dedicated temperature cycling of the sample at the remnant state. As an
ensemble, the nanocrystals exhibits (blocked) superparamagnetic properties. The
magnetic susceptibility of both n- and p-type PbTe in the temperature range
$100 < T < 400$~K has been established. These magnitudes are essential in
proper accounting for the high temperature magnetic susceptibility of PbTe:Cr.

###Complex Dirac-like Electronic Structure in Atomic Site Ordered Rh3In3.4Ge3.6|Aikaterini Flessa Savvidou,Judith K. Clark,Hua Wang,Kaya Wei,Eun Sang Choi,Shirin Mozaffari,Xiaofeng Qian,Michael Shatruk,Luis Balicas###

Complex Dirac-like Electronic Structure in Atomic Site Ordered Rh3In3.4Ge3.6. We report the synthesis via an indium flux method of a novel
single-crystalline compound Rh3In3.4Ge3.6 that belongs to the cubic Ir3Ge7
structure type. In Rh3In3.4Ge3.6, the In and Ge atoms choose to preferentially
occupy, respectively, the 12d and 16f sites of the Im-3m space group, thus
creating a colored version of the Ir3Ge7 structure. Like the other compounds of
the Ir3Ge7 family, Rh3In3.4Ge3.6 shows potential as a thermoelectric displaying
a relatively large power factor, PF ~ 2 mW/cmK2, at a temperature T ~ 225 K
albeit showing a modest figure of merit, ZT = 8 x 10-4, due to the lack of a
finite band gap. These figures might improve through a use of chemical
substitution strategies to achieve band gap opening. Remarkably, electronic
band structure calculations reveal that this compound displays a complex
Dirac-like electronic structure relatively close to the Fermi level. The
electronic structure is composed of several Dirac type-I and type-II nodes, and
even Dirac type-III nodes that result from the touching between a flat band and
a linearly dispersing band. This rich Dirac-like electronic dispersion offers
the possibility to observe Dirac type-III nodes and study their role in the
physical properties of Rh3In3.4Ge3.6 and related Ir3Ge7-type materials.

###Sb concentration dependent Structural and Transport properties of Polycrystalline (Bi1-xSbx)2Te3 Mixed crystal|K. Malik,S. Mahakal,Diptasikha Das,Aritra Banerjee,S. Chatterjee,Anusree Das###

Sb concentration dependent Structural and Transport properties of Polycrystalline (Bi1-xSbx)2Te3 Mixed crystal. (Bi1-xSbx)2Te3 (x=0.60, 0.65, 0.68, 0.70, 0.75 and 0.80) mixed crystals have
been synthesized by solid state reaction. In depth structural, thermal,
transport and electronic properties are reported. Defect and disorder play a
crucial role in structural and transport behaviour. Disorder induced
iso-structural phase transition is observed at x=0.70, which is supported by
the structural and transport properties data. Debye temperature has been
estimated from the powder diffraction data. Differential scanning calorimetry
(DSC) data confirms the glass transition in the material. Low temperature
resistivity data shows Variable range hopping mechanism whereas high
temperature data follows activated behaviour. Activation energy is calculated
from the semiconducting region of resistivity data. Both Hall measurement and
temperature dependent thermopower data (S(T)) confirms that samples are p-type
in nature. Density of state effective mass has been estimated from Pisarenko
relation and corroborated with resistivity data. Thermal conductivity (k) is
estimated using experimentally obtained data. Figure of Merit (ZT) of the
synthesized samples are calculated using resistivity, S(T) and k. Structural
and transport properties are correlated, confirms the transition from disorder
to order state. Defect and disorder are corroborated with structural and
Thermoelectric properties of the synthesized samples.

###Ultralow Thermal Conductivity and Thermoelectric Properties of Bi4GeTe7 with an Intrinsic van der Waal Heterostructure|Niraj Kumar Singh,Ankit Kashyap,Ajay Soni###

Ultralow Thermal Conductivity and Thermoelectric Properties of Bi4GeTe7 with an Intrinsic van der Waal Heterostructure. Ternary chalcogenides, having large crystalline unit cell and van der Waal
stacking of layers, are expected to be poor thermal conductors and good
thermoelectric (TE) materials. We are reporting that layered Bi4GeTe7, with
alternating quintuplet-septuplet layers of Bi2Te3 and Bi2GeTe4, has an ultralow
thermal conductivity, \k{appa}total 0.42 Wm-1K-1 because of high degree of
anharmonicity as estimated from large Gruneisen parameter ({\gamma} 4.07) and
low Debye temperature ({\theta}d 135 K). The electron dominated charge
transport has been realized from the Seebeck coefficient, S - 82 uV/K, at 380
K, and Hall carrier concentration of ne ~ 9.8 x 1019 cm-3 at 300 K. Observation
of weak antilocalization (WAL), due to spin-orbit coupling (SOC) of heavy Bi
and Te, advocate Bi4GeTe7 to be a topological quantum material also. The
cross-sectional transmission electron microscopy images show the inherent
stacking of hetero-layers, which are leading to a large anharmonicity for poor
phonon propagation. Thus, being a poor thermal conductor with a TE figure of
merit, ZT ~ 0.24, at 380 K, the Bi4GeTe7 is a good material for TE
applications.

###Band degeneracy, resonant level formation and low thermal conductivity in dilute In and Ga co-doped thermoelectric compound SnTe|Gaurav Jamwal,Ankit Kumar,Mohd Warish,Shruti Chakravarty,Saravanan Muthiah,Asokan Kandasami,Asad Niazi###

Band degeneracy, resonant level formation and low thermal conductivity in dilute In and Ga co-doped thermoelectric compound SnTe. We report the effect of co-doping of In and Ga at low concentrations on the
structural, electronic, and thermoelectric properties of SnTe based
compositions $Sn_{1.03-2x}In_{x}Ga_{x}Te$ (x = 0, 0.01, 0.02, 0.04) prepared by
the solid-state route and spark plasma sintering (SPS). All compositions formed
in the fcc structure (Fm-3m) with no other impurity phase. The optical band gap
increased with the co-doping, indicative of band convergence effects. First
principle electronic structure calculations showed band convergence and the
formation of resonant levels, due to Ga and In doping respectively. The carrier
concentration increased on hole-doping by In and Ga ions while carrier mobility
decreased due to impurity scattering. The resistivity increased with
temperature, indicative of the degenerate semiconducting character of the
compounds. The Seebeck coefficient of the doped samples increased linearly with
temperature, reaching 85 - 95 ${\mu}$V/K at 783 K. Thermal conductivity
decreased sharply with co-doping, and the lattice thermal conductivity dropped
to 0.42 W$m^{-1}$ $K^{-1}$ above 750 K. The enhanced power factor and low
lattice thermal conductivity on doping resulted in a maximum figure of merit ZT
= 0.34 at 773 K, twice that of the pristine SnTe.

###Janus $β$-PdXY (X/Y = S, Se, Te) Materials with high Anisotropic Thermoelectric Performance|Mukesh Jakhar,Raman Sharma,Ashok Kumar###

Janus $β$-PdXY (X/Y = S, Se, Te) Materials with high Anisotropic Thermoelectric Performance. Two-dimensional (2D) materials have garnered considerable attention as an
emerging thermoelectric (TE) material owing to their unique density of state
(DOS) near the Fermi level. We investigate the TE performance of Janus
$\beta$-PdXY (X/Y=S, Se, Te) monolayer materials as a function of carrier
concentration and mid-temperature range (300 to 800 K) by combining density
functional theory (DFT) and semi-classical Boltzmann transport theory. The
phonon dispersion spectra and AIMD simulations confirm their thermal and
dynamical stability. The transport calculation results reveal the highly
anisotropic TE performance for both n and p-type Janus $\beta$-PdXY monolayers.
Meanwhile, the coexistence of low phonon group velocity and converged
scattering rate leads to lower lattice thermal conductivity (K_l) of 0.80 W/m
K, 0.94 W/m K, and 0.77 W/m K along y-direction for these Janus materials.
While the high TE power factor is attributed to the high Seebeck coefficient
(S) and electrical conductivity, which is due to the degenerate top valance
bands of these Janus monolayers. The combination of lower K_l and high-power
factor at 300K (800 K) leads to an optimal figure of merit (ZT) as 0.68 (2.21),
0.86 (4.09) and 0.68 (3.63) for p-type Janus PdSSe, PdSeTe and PdSTe
monolayers. To capture rational electron transport properties, the effects of
acoustic phonon scattering ($\tau$_ac), impurity scattering ($\tau$_imp), and
polarized phonon scattering ($\tau$_polar) are included in the
temperature-dependent electron relaxation time. These findings indicated that
the Janus $\beta$-PdXY monolayers are promising candidates for TE conversion
devices.

###Ab initio study of NaSrSb and NaBaSb as potential thermoelectric prospects|Chandan Kumar Vishwakarma,Mohd Zeeshan,B. K. Mani###

Ab initio study of NaSrSb and NaBaSb as potential thermoelectric prospects. Zintl phases are excellent thermoelectric prospects to put the waste heat to
good use. In the quest of the same, using first-principles methods combined
with Boltzmann transport theory, we explored two recent phases NaSrSb and
NaBaSb. We found low lattice thermal conductivity of 1.9 and 1.3 W m$^{-1}$
K$^{-1}$ at 300~K for NaSrSb and NaBaSb, respectively, which are of the same
order as other potential Zintl phases such as Sr$_3$AlSb$_3$ and BaCuSb. We
account for such low values to short phonon lifetimes, small phonon group
velocities, and lattice anharmonicity in the crystal structure. The calculated
electrical transport parameters based on acoustic deformation potential,
ionized impurity, and polar optical phonon scattering mechanisms reveal large
Seebeck coefficients for both materials. Further, we obtain a high figure of
merit of ZT$\sim$2.0 at 900~K for \textit{n}-type NaSrSb. On the other hand,
the figure of merit of \textit{n}-type NaBaSb surpasses the unity. We are
optimistic about our findings and believe our work would set a basis for future
experimental investigations.

###An unlikely route to low lattice thermal conductivity: small atoms in a simple layered structure|Wanyue Peng,Guido Petretto,Gian-Marco Rignanese,Geoffroy Hautier,Alexandra Zevalkink###

An unlikely route to low lattice thermal conductivity: small atoms in a simple layered structure. In the design of materials with low lattice thermal conductivity, compounds
with high density, low speed of sound, and complexity at either the atomic,
nano- or microstructural level are preferred. The layered compound Mg$_3$Sb$_2$
defies these prevailing paradigms, exhibiting lattice thermal conductivity
comparable to PbTe and Bi$_2$Te$_3$, despite its low density and simple
structure. The excellent thermoelectric performance ($zT$ $\sim$ 1.5) in
$n$-type Mg$_3$Sb$_2$ has thus far been attributed to its multi-valley
conduction band, while its anomalous thermal properties have been largely
overlooked. To explain the origin of the low lattice thermal conductivity of
Mg$_3$Sb$_2$, we have used both experimental methods and ab initio phonon
calculations to investigate trends in the elasticity, thermal expansion and
anharmonicity of $A$Mg$_2Pn_2$ Zintl compounds with $A$ = Mg, Ca, Yb, and $Pn$
= Sb and Bi. Phonon calculations within the quasi-harmonic approximation reveal
large mode Gr\"uneisen parameters in Mg$_3$Sb$_2$ compared with isostructural
compounds, in particular in transverse acoustic modes involving shearing of
adjacent anionic layers. Measurements of the elastic moduli and sound velocity
as a function of temperature using resonant ultrasound spectroscopy provide a
window into the softening of the acoustic branches at high temperature,
confirming their exceptionally high anharmonicity. We attribute the anomalous
thermal behavior of Mg$_3$Sb$_2$ to the diminutive size of Mg, which may be too
small for the octahedrally-coordinated site, leading to weak, unstable
interlayer Mg-Sb bonding. This suggests more broadly that soft shear modes
resulting from undersized cations provide a potential route to achieving low
lattice thermal conductivity low-density, earth-abundant materials.

###Nanostructuring of Ba8Ga16Ge30 clathrates|Vicente Pachecoa,Raul Cardoso--Gil,Deepa Kasinathan,Helge Rosner,Maik Wagner,Lorenzo Tepech--Carrillo,Wilder Carrillo--Cabrera,Katrin Meier,Yuri Grin###

Nanostructuring of Ba8Ga16Ge30 clathrates. First thermoelectric properties measurements on bulk nanostructured
Ba8Ga16Ge30 clathrate-I are presented. A sol-gel-calcination route was
developed for preparing amorphous nanosized precursor oxides. The further
reduction of the oxides led to quantitative yield of crystalline nanosized
Ba8Ga16Ge30 clathrate-I. TEM investigations show the clathrate nanoparticles
retain the size and morphology of the precursor oxides. The clathrate
nanoparticles contain mainly thin plates (approx. 300 nm x 300 nm x 50 nm) and
a small amount of nanospheres (diameter ~ 10 nm). SAED patterns confirm the
clathrate-I structure type for both morphologies. The powders were compacted
via Spark Plasma Sintering (SPS) to obtain a bulk nano-structured material. The
Seebeck coefficient S, measured on low-density samples (53% of {\delta}x-ray),
reaches -145 {\mu}V/k at 375 {\deg}C. The ZT values are quite low (0.02) due to
the high resistivity of the sample (two orders of magnitude larger than bulk
materials) and the low sample density. The trend of the temperature dependence
of S is in agreement with the values obtained from electronic structure
calculations and semi-classical Boltzmann transport theory within the constant
scattering approximation. The total thermal conductivity (1.61 W/mK), measured
on high density samples (93% of {\delta}x-ray), shows a reduction of 20-25% in
relation to the bulk materials (2.1 W/mK). A further shaping of the sample for
the Seebeck coefficient and electrical conductivity measurements was not
possible due to the presence of cracks. An improvement on the design of the
pressing tools, loading of the sample and profile of the applied pressure will
enhance the mechanical stability of the samples. These investigations are now
in progress.

###On the effectiveness of the thermoelectric energy filtering mechanism in low-dimensional superlattices and nano-composites|Mischa Thesberg,Hans Kosina,Neophytos Neophytou###

On the effectiveness of the thermoelectric energy filtering mechanism in low-dimensional superlattices and nano-composites. Electron energy filtering has been suggested as a promising way to improve
the power factor and enhance the ZT figure of merit of thermoelectric
materials. In this work we explore the effect that reduced dimensionality has
on the success of the energy-filtering mechanism for power factor enhancement.
We use the quantum mechanical non-equilibrium Green's function (NEGF) method
for electron transport including electron-phonon scattering to explore 1D and
2D superlattice/nanocomposite systems. We find that, given identical material
parameters, 1D channels utilize energy filtering more effectively than 2D as
they: i) allow one to achieve maximal power factor for smaller well sizes /
smaller grains (which is needed to maximize phonon scattering), ii) take better
advantage of a lower thermal conductivity in the barrier/boundary materials
compared to the well/grain materials in both: enhancing the Seebeck
coefficient; and in producing a system which is robust against detrimental
random deviations from optimal barrier design. In certain cases we find that
the relative advantage can be as high as a factor of 3. We determine that
energy-filtering is most effective when the average energy of carrier flow
varies the most in the wells and the barriers along the channel, an event which
appears when the energy of the carrier flow in the host material is low and
when the energy relaxation mean-free-path of carriers is short. Although the
ultimate reason these aspects, which cause a 1D system to see greater relative
improvement than a 2D, is the 1D system's van Hove singularity in the
density-of-states, the insights obtained are general and inform
energy-filtering design beyond dimensional considerations.

###Effectiveness of nanoinclusions for reducing bipolar effects in thermoelectric materials|Samuel Foster,Neophytos Neophytou###

Effectiveness of nanoinclusions for reducing bipolar effects in thermoelectric materials. Bipolar carrier transport is often a limiting factor in the thermoelectric
efficiency of narrow bandgap materials (such as Bi2Te3 and PbTe) at high
temperatures due to the introduction of an additional term to the thermal
conductivity and a reduction in the Seebeck coefficient. In this work, we
present a theoretical investigation into the ability of nanoinclusions to
reduce the detrimental effect of bipolar transport. Using the quantum
mechanical non equilibrium Greens function (NEGF) transport formalism, we
simulate electronic transport through two-dimensional systems containing
densely packed nanoinclusions, separated by distances similar to the electron
mean free path. Specifically, considering an n type material, where the bipolar
effect comes from the valence band, we insert nanoinclusions that impose
potential barriers only for the minority holes. We then extract the materials
electrical conductivity, Seebeck coefficient, and electronic thermal
conductivity including its bipolar contribution. We show that nanoinclusions
can indeed have some success in reducing the minority carrier transport and the
bipolar effect on both the electronic thermal conductivity and the Seebeck
coefficient. The benefits from reducing the bipolar conductivity are larger the
more conductive the minority band is to begin with (larger hole mean free path
in particular), as expected. Interestingly, however, the benefits on the
Seebeck coefficient and the power factor are even more pronounced not only when
the minority mean free path is large, but when it is larger compared to the
majority conduction band mean free path. Finally, we extract an overall
estimate for the benefits that nanoinclusions can have on the ZT figure of
merit.

###Synergistic effect of workfunction and acoustic impedance mismatch for improved thermoelectric performance in GeTe/WC composite|Ashutosh Kumar,Preeti Bhumla,Artur Kosonowski,Karol Wolski,Szczepan Zapotoczny,Saswata Bhattacharya,Krzysztof Wojciechowski###

Synergistic effect of workfunction and acoustic impedance mismatch for improved thermoelectric performance in GeTe/WC composite. The preparation of composite materials is promising for concurrent
optimization of electrical and thermal transport properties to realize an
improved thermoelectric (TE) performance. We report the effect of work function
and acoustic impedance mismatch (AIM) on the TE properties of
(1-z)Ge0.87Mn0.05Sb0.08Te/(z)WC composite. In particular, a composite
consisting of Mn and Sb co-doped GeTe as a matrix and WC as a dispersed phase
is prepared, and its structural and TE properties are investigated. The
simultaneous increase in electrical conductivity ({\sigma}) and Seebeck
coefficient ({\alpha}) with WC volume fraction (z) results in an enhanced power
factor ({\alpha}^2{\sigma}) in the composite. The rise in {\sigma} is
attributed to increased carrier mobility in the composite. This is further
established from the work function measurement using the Kelvin probe force
microscopy (KPFM) technique and is also supported by the density functional
theory (DFT) calculations. The difference in elastic properties (sound
velocity) between Ge0.87Mn0.05Sb0.08Te and WC results in a high AIM that leads
to a large interface thermal resistance (Rint) between the phases. The
correlation between Rint and the Kapitza radius results in reduced phonon
thermal conductivity (\kappa_ph) of the composite and is discussed using the
Bruggeman asymmetrical model. The decrease in \kappa_{ph} is further
established using phonon dispersion calculations that indicates the decrease in
phonon group velocity in the composite. The simultaneous effect of enhanced
{\alpha}^2{\sigma} and reduced \kappa_ph results in a maximum figure of merit
(zT) of 1.93 at 773K for (1-z)Ge0.87Mn0.05Sb0.08Te/(z)WC composite having
z=0.010. This study shows promise to achieve higher zTav across a wide range of
composite materials having similar electronic structure and different elastic
properties.

###Thermoelectric films and periodic structures and spin Seebeck effect systems: Facets of performance optimization|Nagaraj Nandihalli###

Thermoelectric films and periodic structures and spin Seebeck effect systems: Facets of performance optimization. The growing market for sensors, internet of things, and wearable devices is
fueling the development of low-cost energy-harvesting materials and systems.
Film based thermoelectric (TE) devices offer the ability to address the energy
requirements by using ubiquitously available waste-heat. This review narrates
recent advancements in fabricating high-performance TE films and superlattice
structures, from the aspects of microstructure control, doping, defects,
composition, surface roughness, substrate effect, interface control,
nanocompositing, and crystal preferred orientation realized by regulating
various deposition parameters and subsequent heat treatment. The review begins
with a brief account of heat conduction mechanism, quantum confinement effect
in periodic layers, film deposition processes, thin film configurations and
design consideration for TE in-plane devices, and characterization techniques.
It then proceeds to alayzing the latest findingd on the TE properties of
Bi2(Te,Se)3 and (Bi,Sb)2Te3, PbTe, GeTe, SnSe, SnTe, Cu2-xSe, and skutterudite
films, including superlattices and the performance of TE generators, sensors,
and cooling devices. Thickness dependent microstructure evolution and TE
characteristics of films in relation to temperature are also analyzed. In the
context of spin Seebeck effect (SSE) based systems, SSE mechanism analysis,
developments in enhancing the spin Seebeck signal since its first observation,
and recent developments are covered from the facets of new system design,
signal collection, magnetic manipulation, interface conditions,
thickness-dependent longitudinal spin Seebeck signal, and length scale of
phonon and magnon transport in longitudinal SSE (LSSE) in different bi-layer
systems. At the end, possible strategies for further enhancing zT of TE films
and spin Seebeck signals of many systems are addressed.