We report the Sr substitution effect in an antiferromagnetic insulator LaMnAsO. The Sr doping limit is $x\sim$ 0.10 under the synthesis conditions, as revealed by x-ray diffractions indicate. Upon Sr doping, the room-temperature resistivity drops by five orders of magnitude down to $\sim$0.01 $Ω\cdot$cm, and the temperature dependence of resistivity shows essentially metallic behavior for $x\geq$0.08. Hall and Seebeck measurements confirm consistently that the insulator-to-metal transition is due to hole doping. Strikingly, the room-temperature Seebeck coefficient for the metallic samples is as high as $\sim240 μ$V/K, making the system as a possible candidate for thermoelectric applications.
The discovery of an intrinsic antiferromagnetic topological insulator (AFM-TI) in MnBi$_2$Te$_4$ has attracted intense attention, most of which lies on its electrical properties. In this paper, we report electronic, thermal, and thermoelectric transport studies of this newly found AFM-TI. The temperature and magnetic field dependence of its resistivity, thermal conductivity, and Seebeck coefficient indicate strong coupling between charge, lattice, and spin degrees of freedom in this system. Furthermore, MnBi$_2$Te$_4$ exhibits a large anomalous Nernst signal, which is associated with non-zero …
We show that synthesis-induced Metal -Insulator transition (MIT) for electronic transport along the orthorombic c axis of FeSb$_{2}$ single crystals has greatly enhanced electrical conductivity while keeping the thermopower at a relatively high level. By this means, the thermoelectric power factor is enhanced to a new record high S$^{2}$$σ$ $\sim$ 8000 $μ$WK$^{-2}$cm$^{-1}$ at 28 K. We find that the large thermopower in FeSb$_{2}$ can be rationalized within the correlated electron model with two bands having large quasiparaticle disparity, whereas MIT is induced by subtle structural difference…
We investigate the low-temperature electrical and thermal transport properties in atomically precise metallic heterostructures involving strongly-correlated electron systems. The model of the Mott-insulator/ band-insulator superlattice was discussed in the framework of the slave-boson mean-field approximation and transport quantities were derived by use of the Boltzmann transport equation in the relaxation-time approximation. The results for the optical conductivity are in good agreement with recently published experimental data on (LaTiO$_3)_N$/(SrTiO$_3)_M$ superlattices and allow us to esti…
We review many-body effects, their microscopic origin, as well as their impact onto thermoelectricity in correlated narrow-gap semiconductors. Members of this class---such as FeSi and FeSb$_2$---display an unusual temperature dependence in various observables: insulating with large thermopowers at low temperatures, they turn bad metals at temperatures much smaller than the size of their gaps. This insulator-to-metal crossover is accompanied by spectral weight-transfers over large energies in the optical conductivity and by a gradual transition from activated to Curie-Weiss-like behaviour in th…
We have investigated the magnetic field-induced metal-insulator transition in the tau-phase organic conductors, which occurs in fields above 35 T, and below 14 K, by magnetization, thermoelectric, and pressure dependent transport methods. Our results show that the transition is a bulk thermodynamic process where a magnetic field-dependent gap opens upon entry into the insulating state. We argue that the transition involves a magnetic field-induced change in the electronic structure.
We study thermoelectric transport at low temperatures in correlated Kondo insulators, motivated by the recent observation of a high thermoelectric figure of merit(ZT) in $FeSb_2$ at $T \sim 10 K$. Even at room temperature, correlations have the potential to lead to high ZT, as in $YbAl_3$, one of the most widely used thermoelectric metals. At low temperature correlation effects are especially worthy of study because fixed band structures are unlikely to give rise to the very small energy gaps $E_g \sim 5 kT$ necessary for a weakly correlated material to function efficiently at low temperature.…
The search for semiconductors with high thermoelectric figure of merit has been greatly aided by theoretical modeling of electron and phonon transport, both in bulk materials and in nanocomposites. Recent experiments have studied thermoelectric transport in ``strongly correlated'' materials derived by doping Mott insulators, whose insulating behavior without doping results from electron-electron repulsion, rather than from band structure as in semiconductors. Here a unified theory of electrical and thermal transport in the atomic and ``Heikes'' limit is applied to understand recent transport e…
Motivated by the high sensitivity to Fermi surface topology and scattering mechanisms in magneto-thermoelectric transport, we have measured the thermopower and Nernst effect on the (011)-plane of the proposed topological Kondo insulator SmB$_6$. These experiments, together with electrical resistivity and Hall effect measurements, demonstrate that the (011)-plane also harbors a metallic surface with the effective mass in the order of 10-10$^2$ $m_0$. The surface and bulk conductances are well distinguished in these measurements and are categorized into metallic and non-degenerate semiconducting…
We study the thermopower of a junction between a metal and a strongly correlated semiconductor. Both in the electronic ferroelectric regime and in the Kondo insulator regime the thermoelectric figures of merit, ZT, of these junctions are compared with that of the ordinary semiconductor. By inserting at the interface one or two monolayers of atoms different from the bulk, with a suitable choice of rare-earth elements very high values of ZT can be reached at low temperatures. The potential of the junction as a thermoelectric device is discussed.
We propose a junction of metal and rare-earth compound semiconductor as the basis for a possible efficient low-temperature thermoelectric device. If an overlayer of rare earth atoms differing from the bulk is placed at the interface, very high values of the figure of merit ZT can be reached at low temperature. This is due to sharp variation of the transmission coefficient of carriers across the junction at a narrow energy range, which is intrinsically linked to the localized character of the overlayer f-orbital.
An influence of the localization of itinerant electrons induced by correlated hopping on the electronic charge and heat transport is discussed for the lightly doped Mott insulator phase of the Falicov-Kimball model. The case of strongly reduced hopping amplitude between the sites with occupied f-electron levels, when an additional band of localized d-electron states could appear on the DOS in the Mott gap, is considered. Due to the electron-hole asymmetry and anomalous features on the DOS and transport function induced by correlated hopping, a strong enhancement of the Seebeck coefficient is o…
In this work, we have investigated the electronic structure and thermoelectric properties of quaternary heusler alloy, FeRuTiSi, using first principle DFT tools implemented in WIEN2k and BoltzTraP code. Electronic structure calculations using TB-mBJ potential shows appearance of flat band at the conduction band edge, thus electron in conduction band have the large effective mass (me*), and therefore mainly contribute for negatively large value of Seebeck coefficient (S). This alloy has indirect band gap of 0.59 eV, and shows the n-type transport behavior. Under the constant relaxation time app…
Topological insulators are frequently also one of the best known thermoelectric materials. It has been recently discovered that in 3D topological insulators each skew dislocation can host a pair of 1D topological states a helical TLL. We derive exact analytical formulas for thermoelectric Seebeck coefficient in TLL and investigate up to what extent one can ascribe the outstanding thermoelectric properties of Bi 2 Te 3 to these 1D topological states. To this end we take a model of a dense dislocation network and find an analytic formula for an overlap between 1D (the TLL) and 3D electronic stat…
We have successfully controlled thermoelectric properties of ZnO by changing carrier concentration using an electric double layer transistor (EDLT) which is a feld effect transistor gated by electrolyte solution. The resistivity and the thermopower decreased abruptly by applying gate voltage larger than a threshold voltage ( 2V), indicating the increase of carrier concentration on the ZnO surface. The temperature dependence of resistivity became metallic, which is characterized by weak temperature dependence of the resistivity, when gate voltage exceeded 2V. Corresponding to the resistivity, t…
The effects of Ir doping on the thermoelectric properties of Pt1-xIrxSb2 (x = 0, 0.01, 0.03, and 0.1) with pyrite structure were studied. Measurements of electrical resistivity rho, Seebeck coefficient S, and thermal conductivity kappa were conducted. The results showed an abrupt change from semiconducting behavior without Ir (x = 0) to metallic behavior at x = 0.01. The sample with x = 0.01 exhibited large S and low rho, resulting in a maximum power factor (S^2/rho) of 43 muW/cmK^2 at 400 K. The peculiar "pudding mold"-type electronic band dispersion could explain the enhanced thermoelectric …
The anomalous temperature variation of the thermoelectric power in the f-electron metallic compounds, namely the sign reversal or the maxima, is sometimes interpreted as resulting from the conduction electrons scattering in Born approximation on the acoustic phonons and on the localized spins in the s-f exchange interaction. The experimenters rely on the results of some theoretical works where such thermoelectric power behavior was obtained within these simple models. In the present paper we prove that neither the electron-phonon scattering nor the magnetic s-f scattering in the Born approxima…
In many strongly correlated electron metals the thermoelectric power has a non-monotonic temperature dependence and values that are orders of magnitude larger than for elemental metals. Kelvin proposed a particularly simple expression for the thermopower in terms of the temperature dependence of the chemical potential. We consider a Hubbard model on an anisotropic triangular lattice at half filling, a minimal effective Hamiltonian for several classes of organic charge transfer salts. The finite temperature Lanczos method is used to calculate the temperature dependence of the thermopower using …
A sizable transverse thermoelectric coefficient N , large to the extent that it potentially serves applications, is predicted to arise, by means of first-principles calculations, in a Skyrmion crystal assumed on EuO monolayer where carrier electrons are introduced upon a quantum anomalous Hall insulating phase of Chern number C = 2. This encourages future experiments to pursue such an effect.
We calculate the thermal and thermoelectric transport coefficients for a two-dimensional spin-density-wave metal in the presence of weak disorder and under an external magnetic field using the Mori-Zwanzig memory matrix formalism. As a consequence, we obtain that although the Seebeck coefficient $S$ displays a relatively conventional linear dependence on the temperature of the system, the Nernst coefficient $ν$ exhibits a clear signature of bad-metallicity within the so-called strange metal regime described by the model. This result agrees qualitatively with experimental transport data obtaine…
Here, we have investigated the thermoelectric properties of FeMnScGa alloy by combined use of full potential linearized augmented plane-wave (FP-LAPW) method and Boltzmann transport theory implemented in Wien2K and BoltzTraP code, respectively. Using the TB-mBJ potential, half-metallic nature is clearly observed with energy band gap of ~0.41 eV for down spin channel. Transport coefficients in 200-1000 K temperature range are investigated under the constant relaxation time approximations (tau=10-14 s). Total contributions to the Seebeck coefficients from up and dn-spin channels are estimated by…
We study the thermoelectric effect of two-dimensional metals on a square lattice within semiclassical Boltzmann transport theory with particular focus on electron-electron scattering. We compute the electrical conductivity and the Seebeck coefficient as a function of band filling and temperature for generically chosen hopping parameters in a two-dimensional tight-binding model. The Boltzmann equation is solved numerically after computing the full collision integral, taking the angular and radial degrees of freedom into account. These degrees of freedom of the collision integral, neglected in t…
MoTe2 is a rare transition-metal ditelluride having two kinds of layered polytypes, hexagonal structure with trigonal prismatic Mo coordination and monoclinic structure with octahedral Mo coordination. The monoclinic distortion in the latter is caused by anisotropic metal-metal bonding. In this work, we have examined the Nb doping effect on both polytypes of MoTe2 and clarified a structural phase diagram for Mo1-xNbxTe2 containing four kinds of polytypes. A rhombohedral polytype crystallizing in polar space group has been newly identified as a high-temperature metastable phase at slightly Nb-r…
Engineered $Na_{0.75}CoO_2$ is considered a prime candidate to achieve high-efficiency thermoelectric systems to regenerate electricity from waste heat. In this work, three elements with outmost electronic configurations, (1) an open d shell (Ni), (2) a closed d shell (Zn), and (3) a half filled f shell (Eu) with maximum unpaired electrons, were selected to outline the dopants' effects on electronic and crystallographic structures of $Na_{0.75}CoO_2$. Systematic $ab$ $initio$ density functional calculations with $DMOL^3$ package showed that the Ni and Zn were more stable when substituting Co w…
On the basis of experimental thermoelectric power results and ab initio calculations, we propose that a metal-insulator transition takes place at high pressure (approximately 6 GPa) in MgV_2O_4.
We compute the thermodynamic properties of the Sachdev-Ye-Kitaev (SYK) models of fermions with a conserved fermion number, $\mathcal{Q}$. We extend a previously proposed Schwarzian effective action to include a phase field, and this describes the low temperature energy and $\mathcal{Q}$ fluctuations. We obtain higher-dimensional generalizations of the SYK models which display disordered metallic states without quasiparticle excitations, and we deduce their thermoelectric transport coefficients. We also examine the corresponding properties of Einstein-Maxwell-scalar theories on black brane geom…
A detailed investigation of the non-equilibrium steady-state electric and thermoelectric transport properties of a quantum dot coupled to the normal metallic and s-wave superconducting reservoirs (N-QD-S) are provided within the Coulomb blockade regime. Using non-equilibrium Keldysh Green's function formalism, initially, various model parameter dependence of thermoelectric transport properties are analysed within the linear response regime. It is observed that the single-particle tunnelling close to the superconducting gap edge can generate a relatively large thermopower and figure of merit. M…
