27 research outputs found
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 SrSnO revealed by Sn-NMR
We have performed Sn-NMR measurements on the antiperovskite oxide
superconductor SrSnO 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 SrSnO
phases. The measurement of the nuclear spin-lattice relaxation rate
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 , attributable to the
presence of a Dirac-electron band.Comment: 5 pages, 4 figure
Penetration depth and gap structure in the antiperovskite oxide superconductor SrSnO revealed by SR
We report a SR study on the antiperovskite oxide superconductor
SrSnO. With transverse-field SR, we observed the increase of the
muon relaxation rate upon cooling below the superconducting transition
temperature K, evidencing bulk superconductivity. The
exponential temperature dependence of the relaxation rate at low
temperatures suggests a fully gapped superconducting state. We evaluated the
zero-temperature penetration depth 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
is larger than those of ordinary
superconductors and is comparable to those of unconventional superconductors.
The relatively high for small carrier density may hint at an
unconventional pairing mechanism beyond the ordinary phonon-mediated pairing.
In addition, zero-field 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
SrSnO, as well as with -wave superconductivity.Comment: 9 pages, 9 figures, to be published in Physical Review
MITOTIC INHIBITOR ACTIVITY OF [6-CLORO-2 (METHYLSUFANYL) PYRIMIDINE -4-AMINE] COBALT (II) COMPLEX
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 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 cm. 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 phonon
mode
Discovery of Superconductivity and Electron-Phonon Drag in the Non-Centrosymmetric Semimetal LaRhGe
We present a comprehensive study of the non-centrosymmetric semimetal
LaRhGe. 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
, higher than typical semimetals. We predict theoretically
the existence of Weyl nodal lines that are protected
by the tetragonal space group. We discover superconductivity for the first time
in this compound with a of 0.39(1) K and of
2.1(1) mT, with evidence from specific heat and transverse-field muon spin
relaxation (). LaRhGe is a weakly-coupled type-I
superconductor, and we find no evidence for time-reversal symmetry breaking in
our zero-field . We study the electrical transport in the normal
state and find an unusual 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
in its normal state are strongly influenced by electron-phonon interactions.
Furthermore, we examine the temperature dependent Raman spectra of LaRhGe
and find that the lifetime of the lowest energy 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