arXiv subjects

Sota Kitamura

Publications and source records attributed to Sota Kitamura.

At least 19 recordsLinked to original sources

arXiv2026

Quantum-geometric shift of quasiequilibrium: Origin of nonreciprocal current driven by quantum-metric dipole

We study nonlinear DC electric transport of quantum-metric origin by combining adiabatic perturbation theory with the nonequilibrium Green function approach. The adiabatic ansatz provides a basis for directly treating a DC electric field in the velocity gauge, rather than introducing it as the zero-frequency limit of an AC field. The resulting adiabatic-basis Hamiltonian takes the same form as in the length gauge, enabling a systematic comparison across different formulations. Applying this fully quantum formulation, we find a longitudinal nonreciprocal current governed by the quantum-metric dipole. The essential ingredient is a quantum correction to the distribution function that is absent in semiclassical treatments. We trace this correction to the finite spread of an electron wave packet during relaxation under a bias field, thereby identifying shifted quasiequilibrium as the physical origin of quantum-metric nonreciprocal transport.

cond-mat.mes-hall↗

arXiv2026

Nonreciprocal current induced by dissipation in time-reversal symmetric systems

We study nonreciprocal current response in noncentrosymmetric crystals under time-reversal symmetry. We reveal that the nonreciprocal current appears in a dissipative system through interband processes. We derive a formula for the nonreciprocal current using the Green's function technique. The nonreciprocal current of the present mechanism turns out to be of $O(1/\tau)$ ($\tau$: the lifetime of Bloch electrons) and arises from the shift of the electron wave packet during the interband processes which has a geometric origin. We present a numerical simulation of the nonreciprocal current in the one-dimensional Rice-Mele model and give its order estimation for nonmagnetic polar semiconductors.

cond-mat.mes-hall↗

arXiv2025

Formulation of the orbital magnetic moment in multiorbital tight-binding models: Application to the inverse Faraday effect

We establish a theoretical formulation of the orbital magnetic moment in multiorbital tight-binding models, focusing on the role of the electric dipole. We demonstrate that the total magnetic moment can be decomposed into several contributions in multiorbital tight-binding models generally. In particular, we reveal that the electric dipole moment of Wannier orbitals also contributes to the orbital magnetic moment, which is not included in the conventional expression for the orbital magnetic moment in lattice systems. The derived formulation for the magnetic moment is applied to the inverse Faraday effect (IFE), a phenomenon where circularly-polarized light induces a magnetic moment. To account for all possible contributions, we adopt an $s$-$p$ tight-binding system as a minimal model for studying the IFE. Using an analytical approach based on the Schrieffer-Wolff transformation, we clarify the physical origins of these contributions. Additionally, we quantitatively evaluate each contribution on an equal footing through a numerical approach based on the Floquet formalism. Our results reveal that the orbital magnetic moment exhibits a significantly larger response compared to the spin magnetic moment, with all contributions to the orbital magnetic moment being comparable in magnitude. These findings highlight the essential role of orbital degrees of freedom in the IFE.

cond-mat.mes-hall↗

arXiv2025

Quantum Optical Spanner: Twisting Superconductors with Vortex Beam via Higgs Mode

Light carrying orbital angular momentum (OAM)--known as vortex beams--has broadened the scope of understanding and applications of light's angular momentum. Optical tweezers using OAM, often referred to as optical spanners, have significantly expanded the tunability of optical manipulation. A key frontier now lies in understanding how vortex beams interact with quantum states of matter. In this work, we numerically investigate the dynamics of a superconductor under vortex beam illumination and demonstrate the transfer of angular momentum from light to the superconducting collective mode, resulting in mechanical rotation. Our findings open a pathway for optical manipulation in the quantum regime, which we term the quantum optical spanner.

cond-mat.str-el↗

arXiv2024

Problem of nonlinear conductivity within relaxation time approximation in noncentrosymmetric insulators

With the recent advancements in laser technology, there has been increasing interest in nonlinear and nonperturbative phenomena such as nonreciprocal transport, the nonlinear Hall effect, and nonlinear optical responses. When analyzing the nonequilibrium steady state, the relaxation time approximation (RTA) in the quantum kinetic equation has been widely used. However, recent studies have highlighted problems with the use of RTA that require careful consideration. In a study published in Phys. Rev. B, $\textbf{109}$, L180302 (2024), we revealed that the RTA has a flaw in predicting finite linear conductivity even for insulators under weak electric fields, and improved the RTA based on the Redfield equation. In this paper, we further extend our approach to nonlinear responses. This approach provides a simple alternative to RTA and is expected to be useful for the study of nonlinear and nonequilibrium phenomena.

cond-mat.mes-hall↗

arXiv2024

Multi-tunneling effect of nonreciprocal Landau-Zener tunneling: Insights from DC field responses

Recent advancements in laser technology have spurred growing interest in nonlinear and nonequilibrium phenomena. Here, we investigate the geometric aspects of quantum tunneling and the nonreciprocal response, particularly focusing on the shift vector, in noncentrosymmetric insulators under a strong DC electric field. In insulators under a strong electric field, electrons undergoing Bloch oscillations interfere with each other by passing through different paths via Landau-Zener tunneling. We found that the interference effect due to multi-tunneling causes the oscillating nonreciprocal response that is significantly amplified with increasing electric field intensity. We also clarified the role of the shift vector in the interference conditions through an analysis of the nonequilibrium steady state. These results will contribute significantly to advancing a systematic understanding of quantum geometric effects in the nonperturbative regime.

cond-mat.mes-hall↗

arXiv2024

Dirac Electrons in AC-Magnetic Fields: $\pi$-Landau Levels and Chiral Anomaly-Induced Homodyne Effect

Floquet engineering, which involves controlling systems through time-periodic driving, is a method for coherently manipulating quantum materials and realizing dynamical states with novel functionalities. Most research in solid-state systems has focused on the use of AC-\textit{electric} fields as the controlling drive. In this study, we investigate the effects of AC-\textit{magnetic} fields on two-dimensional (2D) Dirac electrons and report the emergence of new states and new transport phenomena. In a magnetic field that temporarily changes its direction, the 2D Dirac electrons form a new localized state with a flat band dispersion, dubbed as a $\pi$-Landau level. Its wave function is a superposition of the clockwise and counterclockwise cyclotron orbits with time-periodic amplitudes, resulting in a novel closed trajectory shaped like a figure eight. Then, what would be the counterpart of the Hall effect in AC-magnetic fields? We find that a DC-current in the transverse direction, \textit{i.e.} a homodyne Hall current, is generated when an additional AC-electric field is applied. In the case of Dirac electrons, several electronic states contribute to this phenomenon including the $\pi$-Landau level. However, when the chemical potential $\mu$ is near the Dirac point, the dominant contribution comes from the low-energy electrons and we numerically find the homodyne Hall current to behave as $I_y=-\frac{e}{h}\mu$ per valley and spin. We explain this phenomenon through the high-frequency effective Floquet Hamiltonian which resembles the chiral Landau level Hamiltonian of three-dimensional Weyl Hamiltonian exhibiting chiral anomaly. We discuss the experimental feasibility and conclude that it is possible to realize this new exotic state using techniques such as THz metamaterial enhancement of magnetic fields.

cond-mat.mes-hall↗

arXiv2024

Controllable photocurrent generation in Dirac systems with two frequency drives

We study the bulk photovoltaic effect (BPVE) in Dirac and Weyl semimetals under two-frequency light irradiation. We show that the BPVE emerges for centrosymmetric Dirac and Weyl semimetals in the presence of light fields with frequencies $\Omega$ and $2\Omega$. The BPVE under the two frequency drive involves both shift current contribution independent of the carriers lifetime $\tau$ and the injection current contribution $\propto \tau$. Our calculations indicate that the photocurrent's direction, magnitude and type can be dynamically controlled by tuning parameters of the driving fields. Furthermore, we find that the tilt of the Dirac cone significantly affects the photocurrent, particularly in mirror symmetry-lacking Weyl semimetals, leading to an anisotropic optical response. These findings provide new insights into the dynamic control of photocurrents in topological semimetals, offering promising applications for optoelectronic devices.

cond-mat.mes-hall↗

arXiv2024

Unexpected linear conductivity in Landau-Zener model: limitations and improvements of the relaxation time approximation in the quantum master equation

The nonequilibrium steady states of quantum materials have many challenges. Here, we highlight issues with the relaxation time approximation (RTA) for the DC conductivity in insulating systems. The RTA to the quantum master equation (QME) is frequently employed as a simple method, yet this phenomenological approach is exposed as a fatal approximation, displaying metallic DC conductivity in insulating systems within the linear response regime. We find that the unexpected metallic behavior is caused by the fact that the density matrix in the RTA incompletely incorporates the first order of the external field. To solve this problem, we have derived a new calculation scheme based on the QME that ensure correct behavior in low electric fields. Our method reproduces well the overall features of the exact electric currents in the whole field region. It is not time-consuming, and its application to lattice systems is straightforward. This method will encourage progress in this research area as a simple way to more accurately describe nonequilibrium steady states.

cond-mat.mes-hall↗

arXiv2023

Brillouin zone folding method for quasiperiodic superconductivity in multilayer systems: application to electronic structure and optical responses

We construct an efficient momentum space approach to the superconductivity in quasiperiodic multilayer systems. To this end, we extend the Brillouin zone (BZ) folding method to the superconducting (SC) phases by formulating the gap equation in the momentum space representation with the BZ folding. We show that the physical observables in quasiperiodic multilayers are generally given by so-called quasiperiodic functions. Consequently, there appear SC order parameters with finite momenta in the BZ folding method, which corresponds to the spatial fluctuation of the SC order in quasiperiodic multilayers. We also find a systematic way to compute the physical observables in the BZ folding method with a proper normalization condition using the continued fraction approximation. We apply the BZ folding method to a one-dimensional toy model of quasiperiodic superconductors and demonstrate its numerical efficiency compared to the conventional method based on the real space representation with a large system size. We also study a quasiperiodic bilayer system of Rice-Mele model and an s-wave superconductor to demonstrate an efficient computation of optical responses of a quasiperiodic superconductor with inversion symmetry breaking.

cond-mat.supr-con↗

arXiv2023

Time-dependent Gutzwiller simulation of Floquet topological superconductivity

Periodically driven systems provide a novel route to control the topology of quantum materials. In particular, Floquet theory allows an effective band description of periodically-driven systems through the Floquet Hamiltonian. Here, we study the time evolution of $d$-wave superconductors irradiated with intense circularly-polarized laser light. We consider the Floquet $t$-$J$ model with time-periodic interactions, and investigate its mean-field dynamics by formulating the time-dependent Gutzwiller approximation. We observe the development of the $id_{xy}$-wave pairing amplitude along with the original $d_{x^2-y^2}$-wave order upon gradual increasing of the field amplitude. We further numerically construct the Floquet Hamiltonian for the steady state, with which we identify the system as the fully-gapped $d+id$ superconducting phase with a nonzero Chern number. We explore the low-frequency regime where the perturbative approaches in the previous studies break down, and find that the topological gap of an experimentally-accessible size can be achieved at much lower laser intensities.

cond-mat.str-el↗

arXiv2023

Geometric aspects of nonlinear and nonequilibrium phenomena

We review recent developments in the research of nonlinear and nonequilibrium phenomena in solids focusing on their geometrical aspects. We start with introducing the basic concepts of geometrical phases of Bloch electrons and Floquet theory for periodically driven systems. Then we review recent attempts to engineer topological phases in nonequilibrium matters such as graphene, magnets, and superconductors irradiated with circularly polarized light. We next review a bulk photovoltaic effect of inversion broken materials focusing on the shift current response. The shift current is described with Berry connections and has a close relationship to the modern theory of polarization. We further review recent extensions of the shift current to correlated electron systems. Finally, we explain the geometric diabatic time evolution in the Zener tunneling process and its consequences on nonreciprocal transport.

cond-mat.mes-hall↗

arXiv2023

Photocurrent induced by a bicircular light drive in centrosymmetric systems

A bicircular light (BCL) consists of left and right circularly polarized lights with different frequencies, and draws a rose-like pattern with a rotational symmetry determined by the ratio of the two frequencies. Here we show that an application of a BCL to centrosymmetric systems allows a photocurrent generation through introduction of an effective polarity to the system. We derive formulas for the BCL-induced photocurrent from a standard perturbation theory, which is then applied to a simple 1D model and 3D Dirac/Weyl semimetals. A nonperturbative effect with strong light intensity is also discussed with the Floquet technique.

cond-mat.mes-hall↗

arXiv2022

Nonlinear spin current of photoexcited magnons in collinear antiferromagnets

We study the nonlinear magnon spin current induced by an ac electric field under light irradiation in collinear antiferromagnets with broken inversion symmetry. For linearly polarized light, we find that a dc spin current appears through ``the magnon spin shift current" mechanism, which is driven by a spin polarization generation in the two magnon creation process and has a close relationship to the geometry of magnon bands through Berry connection. For circularly polarized light, a dc spin current appears through ``the spin injection current" mechanism, which is proportional to the relaxation time of magnons and can be large when the magnon lifetime is long. We demonstrate generation of the magnon spin shift and injection currents, based on a few toy models and a realistic model for a multiferroic material M$_2$Mo$_3$O$_8$.

cond-mat.str-el↗

arXiv2022

Gap labeling theorem for multilayer thin film heterostructures

Quasiperiodic systems show a universal gap structure due to quasiperiodicity which is analogous to gap openings at the Brillouin zone boundary in periodic systems. The integrated density of states (IDoS) below those energy gaps are characterized by a few integers, which is known as the ``gap labeling theorem'' (GLT) for quasiperiodic systems. In this study, focusing on multilayer thin film systems such as twisted bilayer graphene and stacked transition metal dichalcogenides, we extend the GLT for multilayer systems of arbitrary dimensions and number of layers, using an approach based on the algebra called ``a noncommutative torus''. We find that the energy gaps and the associated IDoS are generally characterized by $_{DN}C_D$ integer labels in $N$ layer systems in the $D$ dimensions, when the effect of the interlayer coupling can be approximated by a quasiperiodic intralayer coupling for each layer. We demonstrate that the generalized GLT holds for quasiperiodic 1D tight binding models by numerical simulations.

cond-mat.str-el↗

arXiv2022

Thermal Hall responses in frustrated honeycomb spin systems

We study geometrical responses of magnons driven by a temperature gradient in frustrated spin systems. While Dzyaloshinskii-Moriya (DM) interactions are usually incorporated to obtain geometrically nontrivial magnon bands, here we investigate thermal Hall responses of magnons that do no rely on the DM interactions. Specifically, we focus on frustrated spin systems with sublattice degrees of freedom and show that a nonzero Berry curvature requires breaking of an effective $PT$ symmetry. According to this symmetry consideration, we study the $J_1$-$J_2$-$J_2^\prime$ Heisenberg models on a honeycomb lattice as a representative example, and demonstrate that magnons in the spiral phase support the thermal Hall effect once we introduce a magnetic field and asymmetry between the two sublattices. We also show that driving the magnons by a temperature gradient induces spin current generation (i.e., magnon spin Nernst effect) in the $J_1$-$J_2$-$J_2^\prime$ Heisenberg models.

cond-mat.str-el↗

arXiv2021

Optical response of the Leggett mode in multiband superconductors in the linear response regime

We study optical responses of Leggett modes in multiband superconductors in the linear response regime. The Leggett mode is a collective mode unique to multiband superconductors that arises from relative phase fluctuations of superconducting orders for different bands. We use the Ginzburg-Landau (GL) description to study the collective modes in multiband systems. We find that multiband superconductors generally allow a linear coupling between the Leggett mode and external electric fields due to the presence of a cross term between different components of superconducting orders in the GL theory. The presence of a linear coupling for the Leggett mode is in sharp contrast with the absence of that for Higgs (amplitude) modes in single-band superconductors that only support nonlinear optical responses such as third harmonic generation (THG). We further confirm such a linear coupling in multiband superconductors by a more microscopic description based on a diagrammatic approach. We study the collective modes within the random phase approximation (RPA) and compute their contribution to the linear optical conductivity. These findings suggest a new route to observe the Leggett mode by optical absorption.

cond-mat.str-el↗

arXiv2021

Floquet topological $d+id$ superconductivity induced by chiral many-body interactions

We study how a $d$-wave superconductivity is changed when illuminated by circularly-polarised light (CPL) in the repulsive Hubbard model in the strong-coupling regime. We adopt the Floquet formalism for the Gutzwiller-projected effective Hamiltonian with the time-periodic Schrieffer-Wolff transformation. We find that CPL induces a topological superconductivity with a $d+id$ pairing, which arises from the chiral spin coupling and the three-site term generated by the CPL. The latter term remains significant even for low frequencies and low intensities of the CPL. This is clearly reflected in the obtained phase diagram against the laser intensity and temperature for various frequencies red-detuned from the Hubbard $U$, with the transient dynamics also examined. The phenomenon revealed here can open a novel, dynamical way to induce a topological superconductivity.

cond-mat.str-el↗