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Hidetoshi Fukuyama

Publications and source records attributed to Hidetoshi Fukuyama.

At least 19 recordsLinked to original sources
arXiv2026

Thermoelectric and Magnetic Properties in Doped Fe$_2$VAl within a Bipolar Random Anderson Model

We investigate the thermoelectric and magnetic properties of Si-substituted $n$-type and Ti-substituted $p$-type Heusler alloy Fe$_2$VAl using the bipolar random Anderson model, which has been introduced recently to study antisite-defect effects associated with the sign change of the Seebeck coefficient in thermally quenched Fe$_2$VAl. Based on the electronic states of both $n$-type and $p$-type compounds, with the rigid-band shift of the Fermi energy and a temperature-dependent scattering rate taken into account, we elucidate how antisite defects simultaneously influence thermoelectric transport and local magnetic moments. We find that the magnetic moments are enhanced in $n$-type Fe$_2$VAl, whereas they are suppressed in $p$-type Fe$_2$VAl compared with the undoped compound. These contrasting magnetic responses highlight the impact of antisite spin polarization on thermoelectric properties and demonstrate the crucial role of antisite defects in realizing magneto-thermoelectric functionalities in Heusler-type alloys.

cond-mat.mtrl-sci↗
arXiv2026

Phonon-Induced Zero-bias Currents in Solids

Zero-bias current induced by injected phonons in metals and one-dimensional charge density wave (CDW) systems attached on the surface of the piezoelectric substrate is investigated microscopically based on the second order response theory. In contrast to the shift currents discovered by von Baltz and Kraut in which the zero-bias current is induced by AC electric field in systems without inversion symmetry, propagating phonons break the inversion symmetry in the presesnt case. The effects of both deformation potential and piezoelectric potential are taken into account. In the CDW system, zero-bias current appears below the transition temperature and its magnitude strongly depends on the position of the chemical potential. Possible experimental consequences are discussed.

cond-mat.mes-hall↗
arXiv2025

Effects of Antisite Defects on Seebeck Coefficient in Fe_2VAl -- Analyses based on Bipolar Random Anderson Model

A microscopic mechanism is proposed for a dramatic sign change of the Seebeck coefficient from positive to negative sign by the introduction of antisite defects in Fe$_2$VAl based on bipolar random Anderson model (BPRAM), which incorporates hybridization effects between randomly distributed antisites and host bands, where the valence and conduction bands are treated separately due to their separation in momentum space. Applying a self-consistent T-matrix approximation, we find that antisite defects in Fe$_2$VAl induce new states in the band overlap region, resulting in a scattering rate that is higher for hole carriers in the valence band than that for electron carriers in the conduction band, leading to negative Seebeck coefficient. This mechanism of sign change presents a potential new approach for controlling thermoelectric properties in semimetallic systems without changing carrier concentration.

cond-mat.str-el↗
arXiv2025

Sommerfeld-Bethe analysis of ZT in inhomogeneous thermoelectrics

The development of good thermoelectric materials exhibiting high $ZT$ (=$\frac{PF}κ T$) requires maximizing power factor, $PF$, mainly governed by electrons, and minimizing thermal conductivity, $κ$, associated not only with electrons but also with phonons. In the present work, we focus on the GeTe and Mg$_3$Sb$_2$ as high $ZT$ materials with inhomogeneous structures and analyze both electrical conductivity, $L_{11}$, and Seebeck coefficient, $S$, with help of Sommerfeld-Bethe formula, resulting in understanding the temperature dependence of $PF$ and the identification of electrons contribution to thermal conductivity, $κ_{\rm el}$. Comparing the obtained $κ_{\rm el}$ and experimentally measured $κ$, the temperature dependence of phonons contribution to thermal conductivity, $κ_{\rm ph}=κ-κ_{\rm el}$, is inferred and analyzed based on the formula by Holland. Comparison of the GeTe and Mg$_3$Sb$_2$ with different types of crystal structures, i.e., GeTe being of a semiordered zigzag nanostructure like a disrupted herringbone structure while Mg$_3$Sb$_2$ of rather uniform amorphous structure, discloses that size effects on temperature dependence of $κ_{\rm ph}$ is large in the former, while very small in the latter. Hence, it is concluded that not only the size of the grain but also its shape has an important influence on $κ_{\rm ph}$ and then $ZT$.

cond-mat.mtrl-sci↗
arXiv2024

Scaling theory of charge transport and thermoelectric response in disordered 2D electron systems: From weak to strong localization

We develop a new theoretical scheme for charge transport and thermoelectric response in two-dimensional disordered systems exhibiting crossover from weak localization (WL) to strong localization (SL). The scheme is based on the scaling theory for Anderson localization combined with the Kubo-Luttinger theory. Key aspects of the scheme include introducing a unified $β$ function that seamlessly connects the WL and SL regimes, as well as describing the temperature ($T$) dependence of the conductance from high to low $T$ regions on the basis of the dephasing length. We found that the Seebeck coefficient, $S$, behaves as $S\propto T$ in the WL limit and as $S\propto T^{1-p}$ ($p < 1$) in the SL limit, both with possible logarithmic corrections. The scheme is applied to analyze experimental data for thin films of the p-type organic semiconductor poly[2,5-bis(3-alkylthiophen-2-yl)thieno(3,2-b)thiophene] (PBTTT).

cond-mat.mes-hall↗
arXiv2023

Two-band Model with High Thermoelectric Power Factor and Its Application to FeSe Thin Film

We propose a simple theoretical model referred to as the {\it two-band model} to realize both a large Seebeck coefficient and high electrical conductivity, resulting in a high thermoelectric (TE) power factor ($PF$). Using the Kubo--Luttinger linear response theory, we apply this model to the TE response of an FeSe thin film reported by Shimizu {\it et al.} to show a high $PF$ with strong temperature dependence. The effects of superconducting fluctuations and excitonic correlations are found to not be critical.

cond-mat.mtrl-sci↗
arXiv2022

Thermoelectric Effect in Mott Variable-Range Hopping

On the basis of the Kubo-Luttinger linear response theory combined with the scaling theory of Anderson localization predicting the energy dependence of localization length near the mobility edge, we have studied the thermoelectric response of a disordered system exhibiting Mott variable-range hopping (VRH). We found that the low-tempetature Seebeck coefficient for VRH conduction in a $d$-dimensional system varies as $S\propto T^{d/(d+1)}$, which is different from the widely used expression, $S\propto T^{(d-1)/(d+1)}$, based on the energy-independent localization length. It is seen that the low-$T$ behavior of $S(T)$ of the thiospinel CuCrTiS$_4$ is in full agreement with the new scheme.

cond-mat.mtrl-sci↗
arXiv2021

Optimal Thermoelectric Power Factor of Narrow-Gap Semiconducting Carbon Nanotubes with Randomly Substituted Impurities

We have theoretically investigated thermoelectric (TE) effects of narrow-gap single-walled carbon nanotubes (SWCNTs) with randomly substituted nitrogen (N) impurities, i.e., N-substituted (20,0) SWCNTs with a band gap of 0.497 eV. For such a narrow-gap system, the thermal excitation from the valence band to the conduction band contributes to its TE properties even at the room temperature. In this study, the N-impurity bands are treated with both conduction and valence bands taken into account self-consistently. We found the optimal N concentration per unit cell, $c_{\rm opt}$, which gives the maximum power factor ($PF$) for various temperatures, e.g., $PF=$0.30$\rm{W/K^2m}$ with $c_{\rm opt}=3.1\times 10^{-5}$ at 300K. In addition, the electronic thermal conductivity has been estimated, which turn out to be much smaller than the phonon thermal conductivity, leading to the figure of merit as $ZT\sim 0.1$ for N-substituted (20,0) SWCNTs with $c_{\rm opt}=3.1\times 10^{-5}$ at 300K.

cond-mat.mes-hall↗
arXiv2020

Theory of Phason Drag Effect on Thermoelectricity

Lee, Rice and Anderson, in their monumental paper, have proved the existence of a collective mode describing the coupled motion of electron density and phonons in one-dimensional incommensurate charge density wave (CDW) in the Peierls state. This mode, which represents the coherent sliding motion of electrons and lattice distortions and affects low energy transport properties, is described by the phase of the complex order parameter of the Peierls condensate, leading to Fröhlich superconductivity in pure systems. Once spatial disorder is present, however, phason is pinned and system is transformed into an insulating ground state: a dramatic change. Since phason can be considered as an ultimate of phonon drag effect, it is of interest to see its effects on thermoelectricity, which has been studied in the present paper based linear response theory of Kubo and Luttinger. The result indicates that a large absolute value of Seebeck coefficient proportional to the square root of resistivity is expected at low temperatures k_B T/Δ<<1 (Δ: Peierls gap) with opposite sign to the electronic contributions in the absence of Peierls gap.

cond-mat.mtrl-sci↗
arXiv2018

Bipolar thermoelectric effects in semiconducting carbon nanotubes: Description in terms of one-dimensional Dirac electrons

The thermoelectric effects in semiconducting single-walled carbon nanotubes (SWCNTs) are investigated based on the linear response theory combined with the thermal Green's function method. It is shown that the electronic states near the lowest conduction band minimum and the highest valence band maximum can be effectively described in terms of one-dimensional (1D) Dirac electrons to which a theoretical scheme is developed to describe the thermoelectric responses making it possible to study the effects of inter-band impurity scattering and in-gap states. Using the proposed scheme, the bipolar thermoelectric effects (i.e., the sign inversion of the Seebeck coefficient) in semiconducting SWCNTs observed in recent experiments are explained. Moreover, the temperature dependence of the Seebeck coefficient of semiconducting SWCNTs at low temperature is clarified.

cond-mat.mes-hall↗
arXiv2017

Possible High Thermoelectric Power in Semiconducting Carbon Nanotubes ~A Case Study of Doped One-Dimensional Semiconductors~

We have theoretically investigated the thermoelectric properties of impurity-doped one-dimensional semiconductors, focusing on nitrogen-substituted (N-substituted) carbon nanotubes (CNTs), using the Kubo formula combined with a self-consistent $t$-matrix approximation. N-substituted CNTs exhibit extremely high thermoelectric power factor ($PF$) values originating from a characteristic of one-dimensional materials where decrease in the carrier density increase both the electrical conductivity and the Seebeck coefficient in the low-N regime. The chemical potential dependence of the $PF$ values of semiconducting CNTs has also been studied as a field-effect transistor and it turns out that the $PF$ values show a noticeable maximum in the vicinity of the band edges. This result demonstrates that "band-edge engineering" will be crucial for solid development of high-performance thermoelectric materials.

cond-mat.mes-hall↗
arXiv2017

Analytical Solutions for the Surface States of Bi$_{1-x}$Sb$_x$ ($0\le x \lesssim 0.1$)

Analytical solutions for the surface state (SS) of an extended Wolff Hamiltonian, which is a common Hamiltonian for strongly spin-orbit coupled systems, are obtained both for semi-infinite and finite-thickness boundary conditions. For the semi-infinite system, there are three types of SS solutions: (I-a) linearly crossing SSs in the direct bulk band gap, (I-b) SSs with linear dispersions entering the bulk conduction or valence bands away from the band edge, and (II) SSs with nearly flat dispersions entering the bulk state at the band edge. For the finite-thickness system, a gap opens in the SS of solution I-a. Numerical solutions for the SS are also obtained based on the tight-binding model of Liu and Allen [Phys. Rev. B, 52, 1566 (1995)] for Bi$_{1-x}$Sb$_x$ ($0\le x \le 0.1$). A perfect correspondence between the analytic and numerical solutions is obtained around the $\bar{M}$ point including their thickness dependence. This is the first time that the character of the SS numerically obtained is identified with the help of analytical solutions. The size of the gap for I-a SS can be larger than that of bulk band gap even for a "thick" films ($\lesssim 200$ bilayers $\simeq 80$ nm) of pure bismuth. Consequently, in such a film of Bi$_{1-x}$Sb$_x$, there is no apparent change in the SSs through the band inversion at $x\simeq 0.04$, even though the nature of the SS is changed from solution I-a to I-b. Based on our theoretical results, the experimental results on the SS of Bi$_{1-x}$Sb$_x$ ($0\le x \lesssim 0.1$) are discussed.

cond-mat.mes-hall↗
arXiv2017

Nuclear Magnetic Relaxation and Knight Shift Due to Orbital Interaction in Dirac Electron Systems

We study the nuclear magnetic relaxation rate and Knight shift in the presence of the orbital and quadrupole interactions for three-dimensional Dirac electron systems (e.g., bismuth-antimony alloys). By using recent results of the dynamic magnetic susceptibility and permittivity, we obtain rigorous results of the relaxation rates $(1/T_1)_{\rm orb}$ and $(1/T_1)_{\rm Q}$, which are due to the orbital and quadrupole interactions, respectively, and show that $(1/T_1)_{\rm Q}$ gives a negligible contribution compared with $(1/T_1)_{\rm orb}$. It is found that $(1/T_1)_{\rm orb}$ exhibits anomalous dependences on temperature $T$ and chemical potential $μ$. When $μ$ is inside the band gap, $(1/T_1)_{\rm orb} \sim T ^3 \log (2 T/ω_0)$ for temperatures above the band gap, where $ω_0$ is the nuclear Larmor frequency. When $μ$ lies in the conduction or valence bands, $(1/T_1)_{\rm orb} \propto T k_{\rm F}^2 \log (2 |v_{\rm F}| k_{\rm F}/ω_0)$ for low temperatures, where $k_{\rm F}$ and $v_{\rm F}$ are the Fermi momentum and Fermi velocity, respectively. The Knight shift $K_{\rm orb}$ due to the orbital interaction also shows anomalous dependences on $T$ and $μ$. It is shown that $K_{\rm orb}$ is negative and its magnitude significantly increases with decreasing temperature when $μ$ is located in the band gap. Because the anomalous dependences in $K_{\rm orb}$ is caused by the interband particle-hole excitations across the small band gap while $\left( 1/T_1 \right)_{\rm orb}$ is governed by the intraband excitations, the Korringa relation does not hold in the Dirac electron systems.

cond-mat.mes-hall↗
arXiv2017

Lorentz Covariance of Dirac Electrons in Solids: Dielectric and Diamagnetic Properties

We study the electrodynamics of Dirac electrons in solids (e.g., bismuth) by comparing it with quantum electrodynamics (QED). It is shown that Lorentz covariance associated with the Dirac electrons in solids results in a remarkable correlation between the dielectric and diamagnetic properties, leading to a significant enhancement in the permittivity directly linked to the well-known phenomenon of large diamagnetism.

cond-mat.mes-hall↗
arXiv2016

Orbital Magnetism of Bloch Electrons I. General Formula

We derive an exact formula of orbital susceptibility expressed in terms of Bloch wave functions, starting from the exact one-line formula by Fukuyama in terms of Green's functions. The obtained formula contains four contributions: (1) Landau-Peierls susceptibility, (2) interband contribution, (3) Fermi surface contribution, and (4) contribution from occupied states. Except for the Landau-Peierls susceptibility, the other three contributions involve the crystal-momentum derivatives of Bloch wave functions. Physical meaning of each term is clarified. The present formula is simplified compared with those obtained previously by Hebborn et al. Based on the formula, it is seen first of all that diamagnetism from core electrons and Van Vleck susceptibility are the only contributions in the atomic limit. The band effects are then studied in terms of linear combination of atomic orbital treating overlap integrals between atomic orbitals as a perturbation and the itinerant feature of Bloch electrons in solids are clarified systematically for the first time.

cond-mat.mtrl-sci↗
arXiv2015

Solitons in the Crossover between Band Insulator and Mott Insulator: Application to TTF-Chloranil under Pressure

Based on the Phase Hamiltonian, two types of solitons are found to exist in the crossover region between band insulator and Mott insulator in one-dimension. Both of these solitons have fractional charges but with different spins, zero and 1/2, respectively. The results are in accord with the experimental results by Kanoda et al. for TTF-Chloranil under pressure.

cond-mat.mtrl-sci↗
arXiv2014

Transport Properties and Diamagnetism of Dirac Electrons in Bismuth

Bismuth crystal is known for its remarkable properties resulting from particular electronic states, e. g., the Shubnikov-de Haas effect and the de Haas-van Alphen effect. Above all, the large diamagnetism of bismuth had been a long-standing puzzle soon after the establishment of quantum mechanics, which had been resolved eventually in 1970 based on the effective Hamiltonian derived by Wolff as due to the interband effects of a magnetic field in the presence of a large spin-orbit interaction. This Hamiltonian is essentially the same as the Dirac Hamiltonian, but with spatial anisotropy and an effective velocity much smaller than the light velocity. This paper reviews recent progress in the theoretical understanding of transport and optical properties, such as the weak-field Hall effect together with the spin Hall effect, and ac conductivity, of a system described by the Wolff Hamiltonian and its isotropic version with a special interest of exploring possible relationship with orbital magnetism. It is shown that there exist a fundamental relationship between spin Hall conductivity and orbital susceptibility in the insulating state on one hand, and the possibility of fully spin-polarized electric current in magneto-optics. Experimental tests of these interesting features have been proposed.

cond-mat.mes-hall↗