arXiv subjects

Hideaki Maebashi

Publications and source records attributed to Hideaki Maebashi.

15 recordsLinked to original sources

arXiv2026

Quantum-critical transport in marginal Fermi liquids

We use the Kubo response functions to calculate the electrical and thermal conductivity and Seebeck coefficient at low temperatures and frequencies in the quantum-critical region for fermions on a lattice. The theory uses scattering of the fermions with the previously derived collective fluctuations due to topological defects of the quantum XY model coupled to fermions. The microscopic model is applicable to the fluctuations of the loop-current order in cuprates as well as to a class of quasi-two-dimensional heavy-fermion and other metallic antiferromagnets, and proposed recently also for the possible loop-current order in Moiré twisted bi-layer graphene and bilayer WSe$_2$. All these metals have a linear-in-temperature electrical resistivity in the quantum-critical region of their phase diagrams, often termed ``Planckian" resistivity. The solution of the Kubo equation for transport shows that vertex renormalizations to the external fields, beside those caused by Aslamazov-Larkin (A-L) processes, are absent. A-L appears as an Umklapp scattering matrix, which gives a temperature-independent multiplicative factor for the electrical resistivity but does not affect the thermal conductivity. We also show that the mass renormalization which gives a logarithmic enhancement of the marginal Fermi-liquid specific heat does not appear in the electrical resistivity and, more remarkably, in the thermal conductivity. On the other hand the mass renormalization $\propto \ln ω_c/T$ appears in the Seebeck coefficient. We also discuss in detail the conservation laws which play a crucial role in all transport properties. We calculate exactly, the numerical coefficients of the transport properties for a circular Fermi surface. The leading temperature dependences is shown to remain the same for a general Fermi surface, but it is too messy to calculate the numerical coefficient.

cond-mat.str-el↗

arXiv2025

Electrical and thermal magnetotransport and the Wiedemann-Franz law in semimetals with electron-electron scattering

We study the electrical and thermal transport properties and the violation of the Wiedemann-Franz (WF) law of two-carrier semimetals using exact treatments of the Boltzmann equation with the impurity and electron-electron scatterings in a magnetic field. For comparison, we also study those in the case of Baber scattering: a single-carrier system with an impurity scattering and phenomenological momentum-dissipative electron-electron scattering. In both systems, the longitudinal and transverse WF laws, $L = L_{\text{H}} = L_{0}= π^2k_B^2/3e^2$, hold at zero temperature, where the Lorenz ratio $L$ and the Hall Lorenz ratio $L_{\text{H}}$ are ratios of thermal conductivity $κ_{μν}$ to electrical conductivity $σ_{μν}$ divided by temperature. However, the electron-electron scattering makes Lorenz ratios deviate from $L_{0}$ with increasing temperature. To describe the WF law in a magnetic field, we introduce another set of Lorenz ratios, $\widetilde{L}$ and $\widetilde{L}_{\text{H}}$, defined as the ratios of the resistivity and the Hall coefficient to their thermal counterparts. The WF laws for them, $\widetilde{L} = \widetilde{L}_{\text{H}} = L_{0}$, and their violation are helpful for the discussion of $L$ and $L_{\text{H}}$. For Baber scattering, our exact result shows $L_{\text{H}}/L_{0} \sim (L/L_{0})^2$ in a weak magnetic field. In semimetals, the violations of the WF laws are significant, reflecting the different temperature dependence between the electrical and thermal resistivities in a magnetic field. This is because the momentum conservation of the electron-electron scattering has a completely different effect on electrical and thermal magnetotransport. We sort out these behaviors using $\widetilde{L}$ and $\widetilde{L}_{\text{H}}$. We also provide a relaxation time approximation, which is useful for comparing theory and experiment.

cond-mat.str-el↗

arXiv2023

Thermoelectric properties in semimetals with inelastic electron-hole scattering

We present systematic theoretical results on thermoelectric effects in semimetals based on the variational method of the linearized Boltzmann equation. Inelastic electron-hole scattering is known to play an important role in the unusual transport of semimetals, including the broad $T^2$ temperature dependence of the electrical resistivity and the strong violation of the Wiedemann-Franz law. By treating the inelastic electron-hole scattering more precisely beyond the relaxation time approximation, we show that the Seebeck coefficient when compensated depends on the screening length of the Coulomb interaction as well as the Lorenz ratio (the ratio of thermal to electric conductivity due to electrons divided by temperature). It is found that deviations from the compensation condition significantly increase the Seebeck coefficient, along with crucial suppressions of the Lorenz ratio. The result indicates that uncompensated semimetals with the electron-hole scattering have high thermoelectric efficiency when the phonon contribution to thermal conductivity is suppressed.

cond-mat.str-el↗

arXiv2022

Data-driven reconstruction of spectral conductivity and chemical potential from thermoelectric transport data

The spectral conductivity, i.e., the electrical conductivity as a function of the Fermi energy, is a cornerstone in determining the thermoelectric transport properties of electrons. However, the spectral conductivity depends on sample-specific properties such as carrier concentrations, vacancies, charge impurities, chemical compositions, and material microstructures, making it difficult to relate the experimental result with the theoretical prediction directly. Here, we propose a data-driven approach based on machine learning to reconstruct the spectral conductivity and chemical potential from the thermoelectric transport data. Using this machine learning method, we first demonstrate that the spectral conductivity and temperature-dependent chemical potentials can be recovered within a simple toy model. In a second step, we apply our method to experimental data in doped one-dimensional telluride Ta$_4$SiTe$_4$~[T. Inohara, \textit{et al.}, Appl. Phys. Lett. \textbf{110}, 183901 (2017)] to reconstruct the spectral conductivity and chemical potential for each sample. Furthermore, the thermal conductivity of electrons and the maximal figure of merit $ZT$ are estimated from the reconstructed spectral conductivity, which provides accurate estimates beyond the Wiedemann-Franz law. Our study clarifies the connection between the thermoelectric transport properties and the low-energy electronic states of real materials, and establishes a promising route to incorporate experimental data into traditional theory-driven workflows.

cond-mat.stat-mech↗

arXiv2019

Nuclear spin relaxation rate near the disorder-driven quantum critical point in Weyl fermion systems

Disorder such as impurities and dislocations in Weyl semimetals (SMs) drives a quantum critical point (QCP) where the density of states at the Weyl point gains a non-zero value. Near the QCP, the asymptotic low energy singularities of physical quantities are controlled by the critical exponents $ν$ and $z$. The nuclear spin-lattice relaxation rate, which originates from the hyperfine coupling between a nuclear spin and long-range orbital currents in Weyl fermion systems, shows intriguing critical behavior. Based on the self-consistent Born approximation for impurities, we study the nuclear spin-lattice relaxation rate $1/T_1$ due to the orbital currents in disordered Weyl SMs. We find that $(T_1T)^{-1}\sim E^{2/z}$ at the QCP where $E$ is the maximum of temperature $T$ and chemical potential $μ(T)$ relative to the Weyl point. This scaling behavior of $(T_1T)^{-1}$ is also confirmed by the self-consistent $T$-matrix approximation, where a remarkable temperature dependence of $μ(T)$ could play an important role. We hope these results of $(T_1T)^{-1}$ will serve as an impetus for exploration of the disorder-driven quantum criticality in Weyl materials.

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↗

arXiv2017

Nuclear Spin Relaxation Time due to the Orbital Currents in Dirac Electron Systems

The nuclear spin relaxation time $T_1$ is calculated taking account of the contributions from orbital currents of Dirac electrons. We consider a simple model of non-interacting Dirac electron gas in the three-dimensional bulk system. The obtained result shows $T^3$ dependence of $1/T_1$ at temperatures $T$ above the energy gap. This temperature dependence agrees qualitatively with the recent $β$-NMR experiment on the bulk of the topological insulator $\mathrm{Bi}_{0.9}\mathrm{Sb}_{0.1}$.

cond-mat.mes-hall↗

arXiv2013

Structural Evolution of 1D Spectral Function from Low- to High-Energy Limits

By exactly analyzing the spin-1/2 Luttinger liquid (LL) and numerically solving a model of a mobile impurity electron in the LL, we obtain the one-electron spectral function $A(p,ω)$ in a one-dimensional (1D) metal in an entire range of $p$ at zero temperature. For $|p|$ near the Fermi point $p_{\rm F}$, $A(p,ω)$ is featured by two prominent peaks of spinon and (anti)holon representing spin-charge separation, but we also find an additional cusp structure between them. For $|p| \gg p_{\rm F}$, this structure evolves as a main peak in $A(p,ω)$ by swallowing the antiholon mode and its dispersion relation approaches the one of a free electron, implying the existence of an electron excitation in the whole region, but not quite a quasiparticle in the Fermi liquid due to ever existing power-law decay of the excitation.

cond-mat.str-el↗

arXiv2011

Fermi Surface Deformation near Charge-Ordering Transition

We study the deformation of a Fermi surface (FS) near charge-ordering (CO) transition. By applying a fluctuation-exchange approximation to the two-dimensional extended Hubbard model, we show that the FS is largely modified by strong charge fluctuations when the wave number of the CO pattern does not match the nesting vector of the FS in a noninteracting system. We also discuss the enhanced anisotropy in quasiparticle properties in the resultant metallic state.

cond-mat.mtrl-sci↗

arXiv2010

Improvement on the GW$Γ$ Scheme for the Electron Self-Energy and Relevance of the $G_0W_0$ Approximation from this Perspective

Based on an exact functional form derived for the three-point vertex function $Γ$, we propose a self-consistent calculation scheme for the electron self-energy with $Γ$ always satisfying the Ward identity. This scheme is basically equivalent to the one proposed in 2001, but it is improved in the aspects of computational costs and its applicability range; it can treat a low-density electron system with a dielectric catastrophe. If it is applied to semiconductors and insulators, we find that the obtained quasiparticle dispersion is virtually the same as that in the one-shot $GW$ approximation (or $G_0W_0$A), indicating that the $G_0W_0$A actually takes proper account of both vertex and high-order self-energy corrections in a mutually cancelling manner.

cond-mat.mtrl-sci↗

arXiv2009

Enhancement of Spin Susceptibility near Charge-Ordering Transition in a Two-Dimensional Extended Hubbard Model

Based on the non-skeleton diagrammatic expansion satisfying the compressibility and spin-susceptibility sum rules, we investigate static charge and spin responses in a two-dimensional extended Hubbard model with the nearest-neighbor Coulomb repulsion in the vicinity of its charge-ordering transition point. In this expansion, we can calculate approximate charge and spin response functions by systematic inclusion of vertex corrections, from which we obtain the uniform susceptibility equal to the so-called q-limit of the response function and the second-order transition point as a divergent point in the same response function at some finite wave-number vector. It is shown that the reentrant charge-ordering transition, which has already been observed in the random-phase approximation (RPA), remains to take place even though the vertex corrections are included beyond the RPA. As a prominent effect of the vertex corrections, we find that the uniform spin susceptibility is enhanced due to charge fluctuations developing toward the charge-ordering transition. We give a qualitative comparison of this enhanced spin susceptibility with the experimental results on the quasi-two-dimensional organic conductors, together with its explanation in the Landau's Fermi-liquid theory.

cond-mat.str-el↗

arXiv2007

Pseudo-Quantum Criticality in Electron Liquids Exhibited in Expanded Alkali Metals

With paying special attention to the divergence in the compressibility $κ$, we study the Coulombic screening in alkali metals to find singular long-range fluctuations in the electronic polarization originating from this divergence. As a consequence of this singularity, we predict the decrease of the equilibration distance between ions against the increase of $r_s$ the Wigner-Seitz radius of valence electrons, provided that the condition of $2r_c < r_s < 4r_c$ is satisfied with $r_c$ the ion-core radius. This prediction is in good quantitative agreement with the recent experiment on liquid Rb.

cond-mat.other↗

arXiv2002

Crossover Temperature from Non-Fermi Liquid to Fermi Liquid Behavior in Two Types of Impurity Kondo Model

Numerical renormalization-group results on entropy of the anisotropic two-channel Kondo model with the band-width cutoff ($D$) in the presence of a magnetic field ($h$) are obtained to determine crossover temperature from the non-Fermi liquid to Fermi liquid fixed point. It is found that the crossover temperature ($T_{\rm x}$) is given by $T_{\rm x} \equiv {r} T_{\rm K} \sim D(ΔJ/J_{\rm av})^2 e^{-1/J_{\rm av}}$ when $(h /T_{\rm K})^2 \ll r \ll 1 $, where $T_{\rm K}$, $J_{\rm av}$ and $ΔJ$ are the Kondo temperature, the average and difference of the exchange coupling constants, respectively. This result indicates that non-Fermi liquid behavior can be seen even if $ΔJ > T_{\rm K}$. Robust similarities of the crossover behavior in the region around the non-Fermi liquid critical point to that of the two-impurity Kondo model are also discussed.

cond-mat↗

arXiv2001

Huge Enhancement of Impurity Scattering due to Critical Valence Fluctuations in a Ce-Based Heavy Electron System

On the basis of the Ward-Pitaevskii identity, the residual resistivity $ρ_{0}$ is shown to exhibit huge enhancement around the quantum critical point of valence transition in Ce-based heavy electron systems. This explains a sharp peak of $ρ_{0}$ observed in CeCu$_2$Ge$_2$ under the pressure at $P\sim$16GPa where the superconducting trasition temperature also exhibit the sharp peak.

cond-mat.str-el↗