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Register a new userThermoelectric Properties of Ho-doped Bi1-xSbx
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The Seebeck coefficients, electrical resistivities, total thermal conductivities, and magnetization are reported for temperatures between 5 and 350 K for n-type Bi0.88Sb0.12 nano-composite alloys made by Ho-doping at the 0, 1 and 3% atomic levels. The alloys were prepared using a dc hot-pressing method, and are shown to be single phase for both Ho contents with grain sizes on the average of 900 nm. We find the parent compound has a maximum of ZT = 0.28 at 231 K, while doping 1% Ho increases the maximum ZT to 0.31 at 221 K and the 3% doped sample suppresses the maximum ZT = 0.24 at a temperature of 260 K.
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The chiral anomaly is the predicted break down of chiral symmetry in a Weyl semimetal with monopoles of opposite chirality when an electric field parallel to a magnetic field is applied. It occurs because of charge pumping from a positive chirality to a negative chirality monopole. Experimental observation of this fundamental effect has been plagued by concerns about the pathways of the current. Here, we unambiguously demonstrate the thermal analog of the chiral anomaly in topological insulator bismuth-antimony alloys driven into an ideal Weyl semimetal state by a Zeeman field, with the chemical potential pinned at the Weyl points, and in which the Fermi surface has no trivial pockets. The experimental signature is a large enhancement of the thermal conductivity in an applied magnetic field parallel to the thermal gradient that follows the Wiedemann-Franz law above 60 K. Absence of current flow avoids extrinsic effects that plague electrical measurements.
After the classification of topological states of matter has been clarified for non-interacting electron systems, the theoretical connection between gapless boundary modes and nontrivial bulk topological structures, and their evolutions as a function of dimensions are now well understood. However, such dimensional hierarchy has not been well established experimentally although some indirect evidences were reported, for example, such as the half-quantized Hall conductance via quantum Hall effect and extrapolation in the quantum-oscillation measurement. In this paper, we report the appearance of the possible chiral edge mode from the surface state of topological insulators under magnetic fields, confirming the dimensional hierarchy in three dimensional topological insulators. Applying laser pulses to the surface state of Bi1-xSbx, we find that the sign of voltage relaxation in one edge becomes opposite to that in the other edge only when magnetic fields are applied to the topological insulating phase. We show that this sign difference originates from the chirality of edge states, based on coupled time-dependent Poisson and Boltzmann equations.
High mobility phonon-glass semimetal $CuAgSe$ has shown promise in recent years as a potential low-temperature thermoelectric material. It exhibits reasonably strong thermoelectric performance as well as an extremely high carrier mobility, both of which are enhanced when the material is doped with Ni at the Cu sites. The exact mechanism by which these enhancements result; however, is unclear. In order to further investigate the effects of chemical substitution on the materials thermoelectric properties, we have prepared and performed various measurements on $CuAgSe$ samples doped with Co and Cr according to the following compositional formulas: $Cu_{1-x}Co_{x}AgSe$ $(x=0.02, 0.05, 0.10)$ and $Cu_{1-x}Cr_{x}AgSe$ $(x=0.02, 0.05)$. Measurements of temperature and magnetic field dependent thermal conductivity, electrical resistivity, and Seebeck coefficient will be discussed. Our results reveal a remarkable sensitivity of $CuAgSe$s thermoelectric properties to chemical doping in general as well as a particular sensitivity to specific dopants. This demonstrated tunability of $CuAgSe$s various properties furthers the case that high mobility phonon glass-semimetals are strong candidates for potential low temperature thermoelectric applications.
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.
The discovery of Weyl semimetals (WSMs) has fueled tremendous interest in condensed matter physics. WSMs require breaking of either inversion symmetry (IS) or time-reversal symmetry (TRS); they can be categorized into type-I and type-II WSMs, characterized by un-tilted and strongly tilted Weyl cones respectively. Type-I WSMs with breaking of IS or TRS and type-II WSMs with IS breaking have been realized experimentally, but TRS-breaking type-II WSM still remains elusive. In this article, we report an ideal TRS-breaking type-II WSM with only one pair of Weyl nodes observed in the antiferromagnetic topological insulator Mn(Bi1-xSbx)2Te4 under magnetic fields. This state is manifested by a large intrinsic anomalous Hall effect, a non-trivial $mathrm{{pi}}$ Berry phase of the cyclotron orbit and a large positive magnetoresistance in the ferromagnetic phase at an optimal sample composition. Our results establish a promising platform for exploring the physics underlying the long-sought, ideal TRS breaking type-II WSM.
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