We present a study of angle-resolved quantum oscillations of electric and thermoelectric transport coefficients in semi-metallic WTe$_{2}$, which has the particularity of displaying a large B$^{2}$ magneto-resistance. The Fermi surface consists of two pairs of electron-like and hole-like pockets of equal volumes in a "Russian doll" structure. Carrier density, Fermi energy, mobility and the mean-free-path of the system are quantified. An additional frequency is observed above a threshold field and attributed to magnetic breakdown across two orbits. In contrast to all other dilute metals, the Nernst signal remains linear in magnetic field even in the high-field ($ω_cτ\gg 1$) regime. Surprisingly, none of the pockets extend across the c-axis of the first Brillouin zone, making the system a three-dimensional metal with moderate anisotropy in Fermi velocity yet a large anisotropy in mean-free-path.
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: $σ$($m$*)), Seebeck coefficient ($S$), thermal conductivity ($κ_e$), and $ZT$ were calculated by using reduced energy ($ζ$=$E$/$k_B T$), as the functions of $T$, and effective mass ($m$*/$m$). Enhancement of $ZT$ was investigated by contour plots of $T$, and $m$*/$m$ versus $ζ$.
Morphology of the interlayer van der Waals surface and differential tunneling conductance in p-Bi2-xSbxTe3-ySey solid solutions were studied by scanning tunneling microscopy and spectroscopy in dependence on compositions. Topological characteristics of the Dirac fermion surface states were determined. It is shown that the thermoelectric power factor and the material parameter enhance with the shift of the Dirac point to the top of the valence band with increasing of atomic substitution in these thermoelectrics. Correlation between topological characteristics, power factor and material paramete…
An important contribution to the thermoelectric and spin-caloric transport properties in magnetic materials at elevated temperatures is the formation of a spin-disordered state due to local moment fluctuations. This effect has not been largely investigated so far. We focus on various magnetic nanostructures of CrTe in the form of thin layers or nanowires embedded in ZnTe matrix, motivated by the miniaturization of spintronics devices and by recent suggestions that magnetic nanostructures can lead to extraordinary thermoelectric effects due to quantum confinement. The electronic structure of th…
Lifshitz transition, a change in Fermi surface topology, is likely to greatly influence exotic correlated phenomena in solids, such as high-temperature superconductivity and complex magnetism. However, since the observation of Fermi surfaces is generally difficult in the strongly correlated systems, a direct link between the Lifshitz transition and quantum phenomena has been elusive so far. Here, we report a marked impact of the pressure-induced Lifshitz transition on thermoelectric performance for SnSe, a promising thermoelectric material without strong electron correlation. By applying press…
There is currently substantial effort being invested into creating efficient thermoelectric nanowires based on topological insulator chalcogenide-type materials. A key premise of these efforts is the assumption that the generally good thermoelectric properties that these materials exhibit in bulk form will translate into similarly good or even better thermoelectric performance of the same materials in nanowire form. Here, we calculate thermoelectric performance of topological insulator nanowires based on Bi2Te3, Sb2Te3 and Bi2Se3 as a function of diameter and Fermi level. We show that the ther…
We have performed systematic density functional calculations and evaluated thermoelectric properties, See- beck coefficient and anomalous Nernst coefficient of half-Heusler comounds CoMSb(M=Sc, Ti, V, Cr, and Mn). The carrier concentration dependence of Seebeck coefficients in nonmagnetic compounds are in good agreement with experimental values. We found that the half-metallic ferromagnetic CoMnSb show large anomalous Nernst effect originating from Berry curvature at the Brillouin zone boundary. These results help to understanding for the mechanism of large anomalous Nernst coefficient and giv…
We report the results of our experimental studies on the magnetic, transport and thermoelectric properties of the ferromagnetic metal CoMnSb. Sizable anomalous Hall conductivity $σ_{yx}$ and transverse thermoelectric conductivity $α_{yx}$ are found experimentally and comparable in size to the values estimated from density-functional theory. Our experiment further reveals that CoMnSb exhibits $-T\ln T$ critical behavior in $α_{yx}(T)$, deviating from Fermi liquid behavior $α_{yx}\sim T$ over a decade of temperature between 10 K to 400 K, similar to ferromagnetic Weyl and nodal-line semimetals. …
The copper silver selenide has two phases: the low-temperature semimetal phase (α-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 α-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…
The best thermoelectric materials are believed to be heavily doped semiconductors. The presence of a bandgap is assumed to be essential to achieve large thermoelectric power factor and figure of merit. In this work, we study HgTe as an example semimetal with competitive thermoelectric properties. We employ ab initio calculations with hybrid exchange-correlation functional to accurately describe the electronic band structure in conjunction with the Boltzmann Transport theory to investigate the electronic transport properties. We show that intrinsic HgTe, a semimetal with large disparity in its …
In topological n-type thermoelectrics based on $Bi_{2}Te_{3}$ with atomic substitutions Bi $\rightarrow$ In, Te $\rightarrow$ Se, S, the morphology and the surface states of Dirac fermions on the interlayer (0001) surface of van der Waals were studied by scanning tunneling microscopy and spectroscopy (STM/STS) techniques. By the STM method, the dark and light spots on the surface were found, which intensities depend on the composition and thermoelectric properties of solid solutions. The observed surface morphology features in the solid solutions are explained by distortions of surface electro…
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…
We report a transport, thermodynamic, and spectroscopic study of the recently identified topological semiconductor ZrTe$_5$ with a focus on elucidating the connections between its band structure and unusual thermoelectric properties. Using time and angle resolved photoemission spectroscopy (tr-ARPES) we observe a small electronic band gap and temperature dependent Fermi level which traverses from a single valence to conduction band with lowering temperature, consistent with previous reports. This low temperature Fermi surface closely matches that derived from quantum oscillations, suggesting i…
Thermoelectric transport properties of an apparent Fermi liquid: Relation to an analytic anomaly in the density of states and application to hole-doped delafossites
以最近发现的铜铁矿能带结构中的无色散区为动机，研究了一个包含不连续性作为分析反常的自由费米子态密度模型。由此产生的温度依赖性的化学势得到准确和不同的近似方案，然后进行了彻底的讨论。这包括引入一个近似的polylogarithmic差异，这是能够访问的参数范围既不涵盖索末菲展开，也不玻尔兹曼近似。它被发现，费米温度和其他几个温度标度可能是非常低的，从而引起实验观察到的行为不同于费米液体理论所描述的。特别是，两种明显的费米液体行为出现在中间温度。这种行为与最近报道的CuCr（1-x）MgxO 2 [A.迈尼昂等al，Solid State Commun. 149，962（2009）]和CuRh（1-x）MgxO 2 [A.迈尼昂等等人，Phys.Rev.B80，115103（2009）]通过温度无关的关联函数比率近似。以这种方式，确定这些材料的有效态密度以及有效载流子密度。此外，后者的材料的比热的结论得出的分析异常的特殊影响。
Topological semimetals provide new opportunities for exploring new thermoelectric phenomena, because of their exotic and nontrivial electronic structure topology around the Fermi surface. In this study, we report on the discovery of giant transverse and longitudinal magneto-thermoelectric (MTE) effects in Mg3Bi2, which is predicted to be a type-II nodal-line semimetal in the absence of spin-orbit coupling (SOC). The maximum transverse power factor is 2182 μWm^{-1}K^{-2} at 13.5 K and 6 Tesla. The longitudinal power factor reaches up to 3043μWm^{-1}K^{-2} at 15 K and 13 Tesla, which is 20 times…
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 ($α$), and high thermal conductivity ($κ$). The Ge vacancy optimization and crystal field engineering results in an enhanced $α$ via excess Ge …
Using first principles calculations, we study the conduction band alignment, effective mass, and Fermi surface complexity factor of n-type Mg3Sb2-xBix (x = 0, 1, and 2) from the full ab initio band structure. We find that with increasing the Bi content the K and M band minima moves away from the conduction band minimum CB1 while the singly-degenerate $Γ$ band minimum shifts rapidly downward and approaches the conduction band minimum. But the favorable six-fold degenerate CB1 band minimum keeps dominating the conduction band minimum and there is no band crossing between the $Γ$ and CB1 band min…
We study longitudinal electric and thermoelectric transport coefficients of Dirac fermions on a simple lattice model where tuning of a single parameter enables us to change the type of Dirac cones from type-I to type-II. We pay particular attention to the behavior of the critical situation, i.e., the type-III Dirac cone. We find that the transport coefficients of the type-III Dirac fermions behave as the limiting case of neither the type-I nor type-II. On the one hand, the qualitative behaviors of the type-III case are similar to those of the type-I case. On the other hand, the transport coeff…
We investigate the electrical conductivity and thermoelectric effects in topological crystalline insulators in the presence of short- and long-range impurity interactions. We employ the generalized Boltzmann formalism for anisotropic Fermi surface systems. The conductivity exhibits a local minimum as doping varies owing to the Van Hove singularity in the density of states originated from the saddle point in the surface states band structure. Suppression of the interband scattering of the charge carriers at high-energy Dirac points results in a maximum in the electrical conductivity. Whenever t…
Searching for high-performance thermoelectric (TE) materials in the paradigm of narrow-bandgap semiconductors has lasted for nearly 70 years and is obviously hampered by a bottleneck of research now. Here we report on the discovery of a few metallic compounds, TiFexCu2x-1Sb and TiFe1.33Sb, showing the thermopower exceeding many TE semiconductors and the dimensionless figure of merits comparable with the state-of-the-art TE materials. A quasi-linear temperature (T) dependence of electrical resistivity in 2 K - 700 K and the logarithmic T-dependent electronic specific heat at low temperature are…
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…
Universality and the thermoelectric transport properties of a double quantum dot system: Seeking for conditions that improve the thermoelectric efficiency
