arXiv subjects

Masao Ogata

Publications and source records attributed to Masao Ogata.

At least 19 recordsLinked to original sources

arXiv2026

Predicted DC current induced by propagating wave in gapless Dirac materials

In this paper, we show that the application of propagating waves can induce a DC current even in systems with spatial inversion symmetry. We derive the equation for the DC current induced by propagating waves using two methods: perturbation theory and Floquet theory. These two approaches yield consistent results. We then apply the equation to gapless graphene subjected to propagating waves. A nonzero DC current is predicted in graphene with next nearest neighbor hopping terms. Nonperturbative effects arising from a strong wave amplitude are also discussed within the framework of Floquet theory.

cond-mat.mes-hall↗

arXiv2026

Nonperturbative effects in second harmonic generation

Second-harmonic generation (SHG) is a quintessential probe of inversion symmetry breaking in condensed matter. While perturbative $χ^{(2)}$ processes are well-documented, the nonperturbative regime under intense driving remains largely unexplored. In this Letter, we develop a nonperturbative Floquet-Keldysh theory to describe SHG in two-band systems. Our analysis reveals the emergence of two distinct types of nonperturbative saturation: a transition from the conventional $E^2$ scaling to a linear $E$ dependence, and a stronger saturation regime where the SHG response becomes independent of the field amplitude. These behaviors are analytically shown to be governed by one-photon and two-photon resonance processes, respectively. By applying our formalism to a tight-binding model of monolayer GeS, we demonstrate that these specific scaling behaviors are observable in realistic materials and are fully consistent with large-scale numerical Floquet-matrix calculations.

cond-mat.str-el↗

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↗

arXiv2026

Hall effect in topologically trivial isolated flat-band systems

We study the Hall effect in topologically trivial isolated flat-band systems (i.e., flat bands are separated from other bands and have zero Chern number) for a weak magnetic field. In a naive semiclassical picture, the Hall conductivity vanishes when dispersive bands are unoccupied, since there are no mobile carriers. To go beyond the semiclassical picture, we establish a fully quantum mechanical gauge-invariant formula for the Hall conductivity that can be applied to any lattice models. We apply the formula to a general $N+M$-band model with $N$ dispersive bands and $M$-fold degenerate isolated flat bands, and find that when the dispersive bands are unoccupied, the total conductivity takes a universal form consisting of the energy difference between the dispersive and flat bands, and the non-Abelian quantum geometric tensor of the flat bands, which can be nonzero in systems with vanishing Berry curvature. We numerically confirm the Hall effect for isolated flat-band lattice models on the honeycomb lattice ($N=M=1$) and two different Kagome lattices ($N=2$, $M=1$ and $N=1$, $M=2$).

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arXiv2025

Spin-Orbit Coupling Effect on the Seebeck Coefficient in Dirac Electron Systems in $α$-(BETS)$_2$I$_3$

The Seebeck coefficient, $S=L_{12}/(TL_{11})$, which is proportional to a ratio of the thermoelectric conductivity $L_{12}$ to the electric conductivity $L_{11}$ with $T$ being temperature is examined for two-dimensional Dirac electrons in the three-quarter filled organic conductor, $α$-(BETS)$_2$I$_3$, [BETS = BEDT-TSeF = bis(ethylenedithio)tetraselenafulvalene] at ambient pressure.Using a tight-binding model obtained with the first-principles relativistic density-functional theory method [Tsumuraya and Suzumura, Eur. Phys. J. B 94, 17 (2021)], we calculate $S$ in the presence of the impurity and electron--phonon scatterings. We show that $S_x < 0$ and $S_y >0$ at high temperatures, where $S_x$ ($S_y$) denotes $S$ perpendicular (parallel) to the molecular stacking axis. There is a sign change of $S_y$ with increasing $T$. We find that, at low temperatures the absolute value of $S$ is enhanced by the spin-orbit coupling. The Seebeck coefficient is examined by dividing it into components of the conduction and valence bands; we find that the electron and hole contributions compete with each other. Such $T$ dependence of $S$ is clarified using the spectral conductivity, which determines $L_{12}$ and $L_{11}$

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arXiv2025

General Strategy for Large Nernst Coefficient

We propose a general strategy for enhancing the anomalous Nernst coefficient based on the Sommerfeld-Bethe relation. This approach provides a systematic framework for understanding the small anomalous Nernst coefficients typically observed in ferromagnets and identifies conditions under which substantial enhancements can be realized. We further introduce simplified models that exhibit large Nernst coefficients as offering illustrative examples.

cond-mat.mtrl-sci↗

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↗

arXiv2025

Orbital paramagnetism without density of states enhancement in nodal-line semimetal ZrSiS

Unconventional orbital paramagnetism without enhanced density of states was recently discovered in the nodal-line semimetal ZrSiS. We propose a novel interband mechanism, linked to the negative curvature of energy dispersions, which successfully accounts for the observed anomalous response. This negative curvature originates from energy variation along the nodal line, inherent in realistic nodal-line materials. Our results suggest that such orbital paramagnetism provides strong evidence for the presence of nodal lines in ZrSiS, and serves as a hallmark of other nodal-line materials.

cond-mat.mes-hall↗

arXiv2025

Effects of Berry Curvature and Orbital Magnetic Moment in the Magnetothermoelectric Transport of Bloch Electron Systems

Thermoelectric transport coefficients up to linear order in the applied magnetic field are microscopically studied using Kubo-Luttinger linear response theory and thermal Green's functions. We derive exact formulas for the thermoelectric conductivity and thermal conductivity in the limit of small relaxation rates for Bloch electrons in terms of Bloch wave functions, which show that the Sommerfeld-Bethe relationship holds. Our final formula contains the Berry curvature contributions as well as the orbital magnetic moment contributions, that arise naturally from the microscopic theory. We show that generalized $f$-sum rules containing the Berry curvature and orbital magnetic moment play essential roles in taking into account the interband effects of the magnetic field. As an application, we study a model of a gapped Dirac electron system with broken time-reversal symmetry and show the presence of a linear magnetothermopower in such systems.

cond-mat.mes-hall↗

arXiv2025

Symmetry requirements for current-induced spin magnetization specific to chiral crystals: Multipole analysis and the hidden spin glide symmetry

Current-induced spin magnetization (CISM) specific to chiral crystals is microscopically analyzed using multipole theory to identify the necessary hopping and spin-orbit couplings (SOCs). Tight-binding models capturing the essence of chiral crystals are introduced to investigate the multipole degrees of freedom possessed by the Hamiltonian. The results reveal that chiral SOC has a multipole degree of freedom specific to chiral crystals. Subsequently, the CISM is evaluated numerically and analytically. The results show that in addition to the chiral SOC, hopping along the $z$-axis, which is irrelevant from a multipole perspective, is crucial for CISM. This hopping is required to break the combined symmetry of wavevector translation and spin flipping, which we refer to as spin glide symmetry. This confirms that hopping irrelevant to chirality can play a crucial role in physical properties arising from chirality without contradicting the framework of multipole theory.

cond-mat.mes-hall↗

arXiv2025

Elasto-Hall conductivity and the anomalous Hall effect in altermagnets

Altermagnets break time-reversal symmetry, preserve the crystal translation invariance, and have a spin density with $d$-wave, $g$-wave, etc. momentum dependencies which do not contribute to the magnetization. When an $s$-wave spin-density contribution cannot be excluded by symmetry a small magnetization and an anomalous Hall effect (AHE) emerge. However, for so-called "pure" altermagnets, where the $s$-wave component is symmetry forbidden even in the presence of SOC, both the zero-field magnetization and the AHE vanish. We show that altermagnets generally exhibit a non-zero elasto-Hall-conductivity, by which application of strain leads to a non-zero AHE. For pure altermagnets it is the only contribution to the AHE. This elasto-Hall-conductivity is caused by strain coupling to the Berry curvature quadrupole that characterizes altermagnets and allows for the determination of the altermagnetic order using transport measurements that are linear in the electrical field. We further show that the emergence of a non-zero magnetization in the presence of strain arises from a different response function: piezomagnetism. While this magnetization gives rise to an additional contribution to the elasto-Hall conductivity, the corresponding Berry curvature is qualitatively different from the distorted Berry curvature quadrupole originating from the altermagnetic order parameter. This insight also helps to disentangle AHE and weak ferromagnetism for systems with symmetry-allowed $s$-wave contribution. Quantitatively, the elasto-Hall conductivity is particularly pronounced for systems with a Dirac spectrum in the altermagnetic state. The same mechanism gives rise to anomalous elasto-thermal Hall, Nernst, and Ettinghausen effects.

cond-mat.mtrl-sci↗

arXiv2024

Orbital magnetic susceptibility of type-I, II, and III massless Dirac fermions in two dimensions

We study the orbital magnetic susceptibility of tilted massless Dirac fermions in two dimensions. It is well-known that the type-I massless Dirac fermions exhibit divergingly-large diamagnetic susceptibility, whereas less is known about the types II and III cases. We first clarify that the orbital magnetic susceptibility is vanishing for the types II and III in the continuum model. We then compare the three types of Dirac fermions for the lattice models. We employ three tight-binding models with different numbers of Dirac points, all of which are two-band models defined on a square lattice. For all three models, we find that the type-I Dirac fermions show the divergingly-large orbital diamagnetic susceptibility, whereas the type-II Dirac fermions exhibit non-diverging paramagnetic susceptibility. The type-III Dirac fermions exhibit diamagnetism but its susceptibility is small compared with the type-I case.

cond-mat.mes-hall↗

arXiv2024

Microscopic theory of spin Nernst effect

We present the microscopic theory of the spin Nernst effect, which is a transverse spin current directly induced by a temperature gradient, employing the linear response theory with Luttinger's gravitational potential method. We consider a generic, non-interacting electron system with randomly distributed impurities and evaluate the spin current response to the gravitational potential. Our theory takes into account a contribution of the local equilibrium current modified by Luttinger's gravitational potential and is thus consistent with the thermodynamic principle that thermal responses should vanish. The Ward-Takahashi identities ensure that the spin Nernst current is well-behaved at low temperatures in any order of the random impurity potentials. Furthermore, we microscopically derive the spin-current version of Mott's formula, which associates the spin Nernst coefficient with the spin Hall conductivity. The spin-current version of the Středa formula is also discussed. To demonstrate these findings, the spin Nernst current of three-dimensional Dirac electrons is computed. Our theory is general and can therefore be extended to interacting electron systems, where Mott's formula no longer holds.

cond-mat.mes-hall↗

arXiv2024

Chirality-dependent second-order spin current in systems with time-reversal symmetry

Spin currents proportional to the first- and second-order of the electric field are calculated in a specific tight-binding model with time-reversal symmetry. Specifically, a tight-binding model with time-reversal symmetry is constructed with chiral hopping and spin-orbit coupling. The spin conductivity of the model is calculated using the Boltzmann equation. As a result, it is clarified that the first-order spin current of the electric field vanishes, while the second-order spin current can be finite. Furthermore, the spin current changes its sign by reversing the chirality of the model. The present results reveal the existence of spin currents in systems with time-reversal symmetry depending on the chirality of the system. They may provide useful information for understanding the chirality-dependent spin polarization phenomena in systems with time-reversal symmetry.

cond-mat.mes-hall↗

arXiv2024

Phonon Drag Effect in Nernst and Thermal Hall Effects: General Theory and Application to Dilute Metal SrTiO$_{3-δ}$

In magnetic fields, thermal gradient-induced effects such as the Nernst and thermal Hall effects are significantly influenced by phonon drag, which works in conjunction with the force on electrons in a magnetic field. We introduce a method to calculate Nernst and thermal Hall conductivities influenced by phonon drag using linear response theory to treat the magnetic field as a first-order perturbation. Our formula is general enough to apply to various systems in which the Green's functions of electrons and phonons are given. We apply the obtained general theory to the recent experiments of dilute metal SrTiO$_{3-δ}$, known for strong Nernst and thermal Hall effects due to phonon drag. We find good agreement even quantitatively. This is notable as all model parameters are derived from experimental data without adjustable parameters.

cond-mat.mtrl-sci↗

arXiv2024

Interatomic spin-orbit coupling in atomic orbital-based tight-binding models

Interatomic hopping mediated by spin-orbit coupling (SOC) entangles spin, orbital and sublattice degrees of freedom of electrons, leading to the emergence of intriguing phenomena such as novel topological insulators and exotic spin-dependent transport including chirality-induced spin selectivity (CISS). Despite these effects, a comprehensive microscopic formalism to describe the spin-dependent hopping remains insufficiently established. In this study, we systematically investigate SOC hopping by analytically deriving the hopping integrals within a two-center approximation based on atomic orbitals. Introducing independent parameters, or extended Slater-Koster symbols, that characterize SOC hopping, we explicitly determine the form of the hopping for $s$, $p$ and $d$ orbitals in the arbitrary hopping directions. Our formalism is then implemented in tight-binding models on several lattices. Furthermore, we examine the effect of SOC on band dispersion by employing a multipole decomposition for the SOC Hamiltonian, providing a fundamental understanding of SOC-induced phenomena. In particular, we derive an explicit expression for the SOC Hamiltonian that causes unique spin splitting in chiral systems by considering a triangular helical chain. Most importantly, the obtained SOC Hamiltonian does not contain a term that has the symmetry of electric toroidal monopole $G_0$ but rather an electric toroidal quadrupole $G_u$, which is the origin of chirality in this case.

cond-mat.mtrl-sci↗

arXiv2024

Seebeck Effect of Dirac Electrons in Organic Conductors under Hydrostatic Pressure Using a Tight-Binding Model Derived from First Principles

The Seebeck coefficient is examined for two-dimensional Dirac electrons in the three-quarter filled organic conductor alpha-(BEDT-TTF)_2I_3 under hydrostatic pressure, where the Seebeck coefficient is proportional to the ratio of the thermoelectric conductivity to the electrical conductivity. We present an improved tight-binding model in two dimensions with transfer energies determined from first-principles density functional theory calculations with an experimentally determined crystal structure. The temperatutre dependence of the Seebeck coefficient is calculated by adding impurity and electron-phonon scatterings. Noting a zero-gap state due to the Dirac cone, which results in a competition from contributions between the conduction and valence bands, we show positive S_x and S_y at finite temperatures and analyze them in terms of spectral conductivity. The relevance of the calculated S_x (perpendicular to the molecular stacking axis) to the experiment is discussed.

cond-mat.mes-hall↗

arXiv2023

Nonlinear optical response in multiband Dirac-Electron System

We study the dc photocurrent induced by linearly polarized light in a multiband Dirac-electron system, focusing on the organic conductor $α$-(BEDT-TTF)$_2$I$_3$. Utilizing perturbation theory, we predict the dependence of shift current on the frequency of light in photodriven $α$-(BEDT-TTF)$_2$I$_3$. Our findings demonstrate a strong correlation between the frequency of light and both the magnitude and direction of the shift current. Furthermore, we delve into the nonperturbative effects of nonlinear optical responses using Floquet theory and demonstrate how the sign of the optical response changes with increasing light intensity. Our results unveil remarkable optical phenomena in the multiband Dirac-electron system and are anticipated to be observed in future experiments.

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