arXiv subjects

Takahiro Morimoto

Publications and source records attributed to Takahiro Morimoto.

At least 37 records · Page 2Linked to original sources

arXiv2025

Emergent inductors in non-helical magnets

The emergent inductor, which is a concept of an inductor employing quantum mechanics on helical magnets, has been studied actively from both theoretical and experimental aspects. Interestingly, finite inductance has been observed not only in spiral magnetic phases but also across various other magnetic phases although the underlying mechanism behind emergent inductance in non-helical magnets remains unresolved. In this study, we broaden the concept of emergent inductance to encompass non-helical magnets and establish a comprehensive formalism of the emergent inductance generated by magnetization dynamics. Using a diagrammatic approach, we derive the linear response of current density mediated by magnetization dynamics and formulate impedance and inductance for general magnetic materials. We reveal that the emergent inductance is composed of two distinct components, each contributing positively or negatively. Notably, the positive inductance arises from Ohmic dissipation including the effect of the emergent electric field (Berry phase in real space) while the negative inductance arises from the polarization of itinerant electrons (Berry curvature in $k$-$t$ space). We also apply the present method to quasi 1D models and show the numerical results of the inductance, the spin dynamics, the impedance, and the Q-value.

cond-mat.str-el↗

arXiv2025

Odd-parity quadrupole order and induced nonreciprocal transport in the kagome metal CsTi$_3$Bi$_5$ driven by quantum interference

Kagome metals present a fascinating platform of quantum phases thanks to the interplay between the geometric frustration and strong electron correlation. Here, we propose the emergence of the electric odd-parity bond order (BO) that originates from the intra-unit-cell odd-parity configuration in recently discovered kagome metal CsTi$_3$Bi$_5$. The predicted E1u BO is induced by the beyond-mean-field mechanism, that is, the quantum interference among different sublattice spin fluctuations. Importantly, the accompanied nematic deformation of the Fermi surface is just ~1% while the intensity of the quasiparticle interference signal exhibits drastic nematic anisotropy, consistent with the scanning tunneling microscope measurements in CsTi$_3$Bi$_5$. The present odd-parity BO triggers interesting phenomena, such as the non-linear Hall effect and emergent electromagnetism.

cond-mat.str-el↗

arXiv2025

Quantum-metric-induced giant and reversible nonreciprocal transport phenomena in chiral loop-current phases of kagome metals

Rich spontaneous symmetry-breaking phenomena with nontrivial quantum geometric properties in metals represent central issues in condensed matter physics. In this context, the emergence of chiral loop-current order, accompanied by time-reversal symmetry-symmetry breaking in various kagome metals, has garnered significant attention. Particularly noteworthy is the giant electrical magnetochiral anisotropy (eMChA) in CsV3Sb5, which provides compelling evidence of time-reversal-symmetry and inversion-symmetry breakings. However, the underlying essence of this observation has remained obscured due to the lack of theoretical understanding. Here, we reveal that the loop-current order causes giant and reversible eMChA coefficient, $γ_{eM}$, is proportional to the loop-current-induced orbital magnetization $M_{orb}$ times the lifetime of conduction electrons $τ$. In kagome metals, the derived $γ_{eM}$ is substantial and reversible by minute magnetic fields, due to the large $τ(\gg a_0/v_{Fermi})$ and the field-induced reversal of $M_{orb}$. By considering the experimentally observed stripe charge-density wave, the loop-current state becomes non-centrosymmetric, thereby giving rise to the eMChA. Surprisingly, the quantum-metric, which defines a fundamental geometric aspect of Bloch wavefunctions, acquires significant momentum dependence in the loop-current phase, resulting in a dramatic enhancement of eMChA by $\sim100$ times. This research not only clarifies the fundamental symmetry-breaking states in kagome metals, but also opens a new path for exploring quantum-metric-induced phenomena arising from exotic quantum phase transitions in strongly correlated metals

cond-mat.str-el↗

arXiv2025

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 $Ω$ and $2Ω$. The BPVE under the two frequency drive involves both shift current contribution independent of the carriers lifetime $τ$ and the injection current contribution $\propto τ$. 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↗

arXiv2025

Interaction-induced nonlinear magnon transport in noncentrosymmetric magnets

We study the effect of the magnon-magnon interaction on the nonlinear magnon transport. The magnonmagnon interaction induces nonreciprocal magnon decay when the time-reversal symmetry is broken and leads to nonlinear thermal responses of magnons. We construct a theoretical framework to study the nonlinear thermal responses due to the nonreciprocal magnon decay by using the imaginary Dyson equation and quantum kinetic theory, which is then applied to models of one-dimensional (1D) antiferromagnets and 2D honeycomb ferromagnets with Dzyaloshinskii-Moriya interactions. An order estimate shows that the nonlinear thermal response from the present mechanism is feasible for experimental measurement.

cond-mat.str-el↗

arXiv2024

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↗

arXiv2024

Direct current generation by dielectric loss in ferroelectrics

We study direct current (DC) generation induced by microwave irradiation to ferroelectric materials. The DC generation originates from microwave absorption called dielectric loss due to the delay of dielectric response. Such current generation can be formulated as the low-frequency limit of the phonon shift current which arises from an increase of electric polarization accompanying photoexcitation of phonons due to the electron-phonon coupling. To study the DC generation by the dielectric loss, we apply the diagrammatic treatment of nonlinear optical responses to photoexcitations of phonons and derive the general formula for phonon shift current. We then study the DC generation in the low-frequency region and find that the current scales as $\propto ω^2$ for the linearly polarized light and time reversal symmetric systems. We estimate the order of magnitude of the DC generation by dielectric loss, indicating its feasibility for experimental detection in the GHz region.

cond-mat.mes-hall↗

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

Tabular Two-Dimensional Correlation Analysis for Multifaceted Characterization Data

We propose tabular two-dimensional correlation analysis for extracting features from multifaceted characterization data, essential for understanding material properties. This method visualizes similarities and phase lags in structural parameter changes through heatmaps, combining hierarchical clustering and asynchronous correlations. We applied the proposed method to datasets of carbon nanotube (CNTs) films annealed at various temperatures and revealed the complexity of their hierarchical structures, which include elements like voids, bundles, and amorphous carbon. Our analysis addresses the challenge of attempting to understand the sequence of structural changes, especially in multifaceted characterization data where 11 structural parameters derived from 8 characterization methods interact with complex behavior. The results show how phase lags (asynchronous changes from stimuli) and parameter similarities can illuminate the sequence of structural changes in materials, providing insights into phenomena like the removal of amorphous carbon and graphitization in annealed CNTs. This approach is beneficial even with limited data and holds promise for a wide range of material analyses, demonstrating its potential in elucidating complex material behaviors and properties.

cond-mat.mtrl-sci↗

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↗

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

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

Giant Magnetochiral Anisotropy in Weyl-semimetal WTe2 Induced by Diverging Berry Curvature

The concept of Berry curvature is essential for various transport phenomena. However, an effect of the Berry curvature on magnetochiral anisotropy, i.e. nonreciprocal magneto-transport, is still elusive. Here, we report the Berry curvature originates the large magnetochiral anisotropy. In Weyl-semimetal WTe2, we observed the strong enhancement of the magnetochiral anisotropy when the Fermi level is located near the Weyl points. Notably, the maximal figure of merit $\barγ$ reaches $1.2\,{\times}10^{-6} \rm{m^2T^{-1}A^{-1}}$, which is the largest ever reported in bulk materials. Our semiclassical calculation shows that the diverging Berry curvature at the Weyl points strongly enhances the magnetochiral anisotropy.

cond-mat.mtrl-sci↗

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

Effects of relaxation on photovoltaic effect and possibility of photocurrent within transparent region

We theoretically study photocurrents in metals that break both inversion $\mathcal{P}$ and $\mathcal{T}$ symmetries within the transparent region. We find that the system under the ac electric fields is well described with an effective Hamiltonian and the photocurrent is of the order of $\mathcal{O}(ω_J/γ)$ if the frequency of the induced current $ω_J$ and the scattering rate $γ$ satisfy $ω_J/γ\ll 1$, and vanishes in the limit of $ω_J/γ\to 0$. On the other hand, the effective Hamiltonian description indicates that nonvanishing photocurrent can appear even in the transparent region if the system is thin enough compared to the mean free path in the direction of the induced current (where $γ$ can be effectively regarded as 0). Candidate materials for the such photovoltaic effect within the transparent region include multiferroics breaking both $\mathcal{P}$ and $\mathcal{T}$ symmetries.

cond-mat.mes-hall↗

arXiv2022

Theory of shift heat current and its application to electron-phonon coupled systems

We propose a heat current analog of the shift current, "shift heat current". We study nonlinear heat current responses to an applied ac electric field by a diagrammatic method and derive a microscopic expression for the second order dc heat current response. As a result, we find that the shift heat current is related to the shift vector, a geometric quantity that also appears in the expression for the shift current. The shift heat current directly depends on and can be controlled through the chemical potential. In addition, we apply the diagrammatic method to electron-phonon coupled systems, and we find that even if only the phonons are excited by an external field, the amplitude of the shift heat current is determined by the energy scale of electrons, not of phonons.

cond-mat.mtrl-sci↗

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↗

arXiv2022

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↗