Browsing by Author "Leithe-Jasper, Andreas"

Filter results by typing the first few letters
Now showing 1 - 2 of 2
  • ItemOpen Access
    Controlling structural instability in the Cu$_{12-x}$Ni$_x$Sb$_4$S$_{13}$ ($x$\,=\,0, 0.05, 0.2, 0.5) tetrahedrites
    (Technische Universität Bergakademie Freiberg, 2026-01-08) Bolielyi, Oleksandr; Dobrozhan, Oleksandr; Levytskyi, Volodymyr; Tsirlin, Alexander A.; Pshenychnyi, Roman; Leithe-Jasper, Andreas; Gumeniuk, Roman
    Cu$_{12-x}$Ni$_x$Sb$_4$S$_{13}$ ($x\,=\,0, 0.05, 0.2, 0.5$) tetrahedrites were synthesized by the polyol method. Measurements of magnetic susceptibility, specific heat capacity as well as electrical and thermal transport properties indicated a structural phase transition (SPT) of 1$^{\mathrm{st}}$ order at $T_k\,=\,75-85$\,K, which is accompanied by strong changes in electronic density of states (EDOS) and entropy for $x\,=\,0, 0.05, 0.2$ whereas only weak changes were observed at $x\,=\,0.5$. However, further temperature dependent synchrotron high-resolution powder X-ray diffraction (HR PXRD) studies indicated compounds with Ni-content $x\,=\,0$ and 0.5 to remain body-centered cubic [space group (SG) $I\bar{4}3m$, $a\,\approx\,10.4(1)$\,\AA] down to 10\,K, whereas crystal structures of $x\,=\,0, 0.05$ and 0.2 became tetragonal, (SG $P\bar{4}c2$, $a_\mathrm{tetr}\,\approx\,a_\mathrm{cub}\sqrt{2}$, $c_\mathrm{tetr}\,\approx\,a_\mathrm{cub}$) below $T_k$. Comparing changes of EDOS at the Fermi level [$\Delta N^{(E_\mathrm{F})}$] deduced from magnetic and thermodynamic data with those calculated from density functional theory (DFT) assuming the validity of the rigid-band approximation, we found $\Delta N^{(E_\mathrm{F})}_\mathrm{theor}\,\approx\,0$ when compared with cubic and tetragonal arragements for Cu$_{11.5x}$Ni$_{0.5}$Sb$_4$S$_{13}$, which indicate the latter to be unstable. This study shows that SPT in tetrahedites is facilitated by the presence of minor amount of a dopant.
  • ItemOpen Access
    Multiple superconducting transitions in Yb$_{3+x}$Co$_4$Sn$_{13-x}$
    (Technische Universität Bergakademie Freiberg, 2025-07-31) Meret, Orlob; Levytskyi, Volodymyr; Tsirlin, Alexander; Kundys, Bohdan; Sturm, Elena; Leithe-Jasper, Andreas; Gumeniuk, Roman
    Abstract of the accompanying article: In the Yb-Co-Sn system a Remeika phase Yb$_{3+x}$Co$_{4}$Sn$_{13-x}$ is found with solid solution extending within $0\le\,x\,\le0.5$. All these compounds crystallize with primitive cubic [space group $Pm\bar{3}n$, $a\approx9.54$ \AA] strongly disordered Sc$_{3}$Ir$_4$Si$_{13+x}$ type. Stoichiometric Yb$_3$Co$_4$Sn$_{13}$ is a superconductor with critical temperature $T_c\,=\,3.1(2)$\,K, lower- [$B_\mathrm{c1}=1.90(2)$\,mT] and upper [$B_\mathrm{c2}=2.16(5)$\,T] critical fields. The specific heat jump $\Delta c_p/\gamma T_c\,=\,1.72(9)$ together with the fact that $B_\mathrm{c2}(T_c)$ dependence is described by the Werthamer-Helfand-Hohenberg (WHH) model hint towards a conventional mechanism with a weak electron-phonon coupling. Although a more precise analysis is hampered by the presence of multiple superconducting transitions observed in the specific heat of this Remeika phase, the non-stoichiometric Yb$_{3.2}$Co$_4$Sn$_{12.8}$ reveals $T_c\,=\,2.4(2)$\,K and $B_\mathrm{c2}=4.79(9)$\,T exceeding the classical Pauli limit. Strong electron-phonon coupling in this superconductor is confirmed by the high value of the $\lambda_\mathrm{AD}=0.92(2)$ parameter determined from the Allen-Dynes formula. However, the electronic specific heat of Yb$_{3.2}$Co$_4$Sn$_{12.8}$ follows the exponential law [$c_\mathrm{el}(T)\propto e^{-\Delta(0)/k_\mathrm{B}T}$] and can be described by the $\alpha$-model [$\alpha\equiv\Delta(0)/k_\mathrm{B}T_c=2$] indicating this stannide to be a conventional BCS-superconductor. Both Yb$_{3+x}$Co$_{4}$Sn$_{13-x}$ ($x=0, 0.2$) compounds reveal complex phonon spectra with possible `rattling' behavior. They are also found to be metallic systems, some aspects of which can be described by a free electron gas model.