Optimization of Thermoelectric Chalcogenides

Thermoelectric (TE) materials can convert heat into electricity when a temperature gradient is applied (Seebeck effect), and can pump heat from the cold end to the hot end using electricity (Peltier effect). These materials have shown improved properties in recent years, and promising bulk materials have been uncovered by many research groups. Chalcogenide containing materials are the leading TEs today, and examples uncovered by the Kleinke group include Tl4(Zr,Hf)Te4, Ba3Cu16–xS11–yTey, and BaCu6-x(S,Se)Te6. Initial evaluation of cold-pressed pellets of these materials showed potential for TE application, with a high figure-of-merit, zT, which relates to the efficiency of a material in converting heat into electricity. This zT is a dimensionless figure composed of heat and electrical transport properties of a material through the equation: zT=〖Tα〗^2 σκ^(-1), where α is the Seebeck coefficient, σ the electrical conductivity, and κ the thermal conductivity, measured at a specific temperature (T). A zT value above unity is desired for a bulk material to be used for power generation. The biggest challenge in optimizing TE materials is decoupling the properties contributing to zT, and tuning the properties in a way that improves the zT overall. In this work, it is demonstrated that the zT value for a material can be improved through doping and hot pressing to close to ideal density. Adding Nb to Tl4ZrTe4 improved the zT compared to the ternary sample by enhancing the electrical conductivity. The zT of the samples of nominal composition "Tl4ZrNb0.04Te4" and Tl4Zr1.03Te4 amounted to 0.064 and 0.042 at ~500 K, respectively. Properties of undoped Tl4HfTe4 were also compared to those of Tl4Zr1.03Te4, and Tl4HfTe4 had a higher zT of 0.08 at ~480 K, which is however still too low for any application. Varying the x and y values in Ba3Cu16–xS11–yTey achieved the highest zT for Ba3Cu15.1S8Te3, amounting to 0.78 at ~780 K. However, after six repeat electrical property measurements, these samples showed color change, which was confirmed, via EDX measurement, to be the result of Cu atoms migrating across the material. In contrast, BaCu6-x(S,Se)Te6 showed great stability after repeat measurements, which is attributed to localized mobility of Cu atoms. The properties of BaCu6-x(S,Se)Te6 were measured, and zT values of 0.52 at 580 K and 0.81 at 600 K were obtained for BaCu5.9STe6 and BaCu5.9SeTe6, respectively.4 month

Normal-state properties of the antiperovskite oxide Sr3−x_{3-x}SnO revealed by 119^{119}Sn-NMR

We have performed $^{119}$Sn-NMR measurements on the antiperovskite oxide superconductor Sr$_{3-x}$SnO to investigate how its normal state changes with the Sr deficiency. A two-peak structure was observed in the NMR spectra of all the measured samples. This suggests that the phase separation tends to occur between the nearly stoichiometric and heavily Sr-deficient Sr$_{3-x}$SnO phases. The measurement of the nuclear spin-lattice relaxation rate $1/T_1$ indicates that the Sr-deficient phase shows a conventional metallic behavior due to the heavy hole doping. In contrast, the nearly stoichiometric phase exhibits unusual temperature dependence of $1/T_1$, attributable to the presence of a Dirac-electron band.Comment: 5 pages, 4 figure

Penetration depth and gap structure in the antiperovskite oxide superconductor Sr3−x_{3-x}SnO revealed by μ\muSR

We report a $\mu$SR study on the antiperovskite oxide superconductor Sr$_{3-x}$SnO. With transverse-field $\mu$SR, we observed the increase of the muon relaxation rate upon cooling below the superconducting transition temperature $T_{\mathrm{c}}=5.4$ K, evidencing bulk superconductivity. The exponential temperature dependence of the relaxation rate $\sigma$ at low temperatures suggests a fully gapped superconducting state. We evaluated the zero-temperature penetration depth $\lambda(0)\propto1/\sqrt{\sigma(0)}$ to be around 320-1020 nm. Such a large value is consistent with the picture of a doped Dirac semimetal. Moreover, we revealed that the ratio $T_{\mathrm{c}}/\lambda(0)^{-2}$ is larger than those of ordinary superconductors and is comparable to those of unconventional superconductors. The relatively high $T_{\mathrm{c}}$ for small carrier density may hint at an unconventional pairing mechanism beyond the ordinary phonon-mediated pairing. In addition, zero-field $\mu$SR did not provide evidence of broken time-reversal symmetry in the superconducting state. These features are consistent with the theoretically proposed topological superconducting state in Sr$_{3-x}$SnO, as well as with $s$-wave superconductivity.Comment: 9 pages, 9 figures, to be published in Physical Review

Signature of an Ultrafast Photo-Induced Lifshitz Transition in the Nodal-Line Semimetal ZrSiTe

Here we report an ultrafast optical spectroscopic study of the nodal-line semimetal ZrSiTe. Our measurements reveal that, converse to other compounds of the family, the sudden injection of electronic excitations results in a strongly coherent response of an $A_{1g}$ phonon mode which dynamically modifies the distance between Zr and Te atoms and Si layers. "Frozen phonon" DFT calculations, in which band structures are calculated as a function of nuclear position along the phonon mode coordinate, show that large displacements along this mode alter the material's electronic structure significantly, forcing bands to approach and even cross the Fermi energy. The incoherent part of the time domain response reveals that a delayed electronic response at low fluence discontinuously evolves into an instantaneous one for excitation densities larger than $3.43 \times 10^{17}$ cm$^{-3}$. This sudden change of the dissipative channels for electronic excitations is indicative of an ultrafast Lifshitz transition which we tentatively associate to a change in topology of the Fermi surface driven by a symmetry preserving $A_{1g}$ phonon mode

Discovery of Superconductivity and Electron-Phonon Drag in the Non-Centrosymmetric Semimetal LaRhGe3_3

We present a comprehensive study of the non-centrosymmetric semimetal LaRhGe$_3$. Our transport measurements reveal evidence for electron-hole compensation at low temperatures, resulting in a large magnetoresistance of 3000% at 1.8 K and 14 T. The carrier concentration is on the order of $10^{21}\rm{/cm}^3$, higher than typical semimetals. We predict theoretically the existence of $\textit{almost movable}$ Weyl nodal lines that are protected by the tetragonal space group. We discover superconductivity for the first time in this compound with a $T_{\text c}$ of 0.39(1) K and $B_{\rm{c}}(0)$ of 2.1(1) mT, with evidence from specific heat and transverse-field muon spin relaxation ($\mu \rm{SR}$). LaRhGe$_3$ is a weakly-coupled type-I superconductor, and we find no evidence for time-reversal symmetry breaking in our zero-field $\mu \rm{SR}$. We study the electrical transport in the normal state and find an unusual $\sim T^{3}$ dependence at low temperature while Seebeck coefficient and thermal conductivity measurements reveal a peak in the same temperature range. We conclude that the transport properties of LaRhGe$_3$ in its normal state are strongly influenced by electron-phonon interactions. Furthermore, we examine the temperature dependent Raman spectra of LaRhGe$_3$ and find that the lifetime of the lowest energy $A_1$ phonon is dominated by phonon-electron scattering instead of anharmonic decay

Superconductivity in (Ba,K)SbO3

Funding: This research was carried out in part due to funding from the Max Planck-UBC-UTokyo Centre for Quantum Materials. Part of the research described in this paper was performed at the Canadian Light Source, a national research facility of the University of Saskatchewan, which is supported by the Canada Foundation for Innovation, the Natural Sciences and Engineering Research Council, the National Research Council, the Canadian Institutes of Health Research, the Government of Saskatchewan and the University of Saskatchewan. We thank the Science and Technology Facilities Council ISIS facility for the provision of beamtime.(Ba,K)BiO3 constitute an interesting class of superconductors, where the remarkably high superconducting transition temperature Tc of 30 K arises in proximity to charge density wave order. However, the precise mechanism behind these phases remains unclear. Here, enabled by high-pressure synthesis, we report superconductivity in (Ba,K)SbO3 with a positive oxygen–metal charge transfer energy in contrast to (Ba,K)BiO3. The parent compound BaSbO3−δ shows a larger charge density wave gap compared to BaBiO3. As the charge density wave order is suppressed via potassium substitution up to 65%, superconductivity emerges, rising up to Tc = 15 K. This value is lower than the maximum Tc of (Ba,K)BiO3, but higher by more than a factor of two at comparable potassium concentrations. The discovery of an enhanced charge density wave gap and superconductivity in (Ba,K)SbO3 indicates that strong oxygen–metal covalency may be more essential than the sign of the charge transfer energy in the main-group perovskite superconductors.Publisher PDFPeer reviewe