arXiv subjects

Shintaro Takayoshi

Publications and source records attributed to Shintaro Takayoshi.

At least 19 recordsLinked to original sources

arXiv2026

Prominent Dimensional Effects on High-order Harmonic Generation in Strongly Correlated Electron Systems

Dimensionality strongly affects elementary excitations in correlated quantum materials, yet its impact on extreme nonlinear optical responses remains largely unexplored. Here, we combine high-harmonic generation (HHG) experiments on quasi-one-dimensional SrCuO$_2$ and quasi-two-dimensional Pr$_2$CuO$_4$ with nonequilibrium simulations of Hubbard models. We find a pronounced dimensional contrast: SrCuO$_2$ exhibits a robust plateau-like high-harmonic spectrum with weak temperature dependence, whereas Pr$_2$CuO$_4$ shows a monotonic decrease in harmonic yield and strong thermal suppression, especially at higher harmonics. The simulations qualitatively reproduce these trends and identify dimensionality-dependent doublon--holon dephasing, governed by spin--charge coupling, as their microscopic origin. These results establish dimensionality and spin--charge coupling as key control parameters for extreme nonlinear optical responses in correlated insulators.

cond-mat.str-el↗

arXiv2026

Spinon shift current in a noncentrosymmetric quantum spin chain

We theoretically study direct current generation in a quantum spin chain induced by spinon excitations by light irradiation. We consider a $S=1/2$ one-dimensional (1D) antiferromagnetic XXZ model with magnetoelectric coupling that describes multiferroics with broken inversion symmetry. We perform the real-time simulation using infinite time-evolving block decimation, and demonstrate the direct current generation under light irradiation. By comparing the second order nonlinear conductivity and the two-spinon excitation spectra of a 1D XXZ model, we confirm that the spinon excitations are the origin for the direct current generation in the quantum spin chain. We find that the bulk photovoltaic effect is driven by electric polarization carried by the spinons through the shift current mechanism, and thus is regarded as ``the spinon shift current.''

cond-mat.str-el↗

arXiv2026

HPC-Driven Modeling with ML-Based Surrogates for Magnon-Photon Dynamics in Hybrid Quantum Systems

Simulating hybrid magnonic quantum systems remains a challenge due to the large disparity between the timescales of the two systems. We present a massively parallel GPU-based simulation framework that enables fully coupled, large-scale modeling of on-chip magnon-photon circuits. Our approach resolves the dynamic interaction between ferromagnetic and electromagnetic fields with high spatiotemporal fidelity. To accelerate design workflows, we develop a physics-informed machine learning surrogate trained on the simulation data, reducing computational cost while maintaining accuracy. This combined approach reveals real-time energy exchange dynamics and reproduces key phenomena such as anti-crossing behavior and the suppression of ferromagnetic resonance under strong electromagnetic fields. By addressing the multiscale and multiphysics challenges in magnon-photon modeling, our framework enables scalable simulation and rapid prototyping of next-generation quantum and spintronic devices.

quant-ph↗

arXiv2025

Geometric Effects on Tunneling in Driven Quantum Systems

We review quantum tunneling provoked by external field driving, focusing on the role of geometric effects. The discussion begins with an overview of tunneling phenomena, including the Landau-Zener model and the Schwinger effect, both of which are essential frameworks to describe the generation of elementary excitation of the system. We also refer to the relation between the modern theory of polarization and the geometry of the system, and introduce the shift vector via adiabatic perturbation theory. Then we introduce the twisted Landau-Zener model and shown how the shift vector modulates tunneling probability, followed by several illustrative applications of this model. We also explain the Keldysh crossover, which is the crossover from a quantum tunneling regime to photon absorption regime in driven systems.

cond-mat.str-el↗

arXiv2024

Many-body Keldysh Crossover in the DC-driven Haldane Spin Chain

We theoretically study nonlinear processes driven by a DC spin-electric field in the antiferromagnetic spin-1 Heisenberg model starting from the ground state in the Haldane phase. The DC spin-electric field generates finite spin current and accumulation since the symmetry protected topological order is destroyed by the field. We find two microscopic mechanisms responsible for the breakdown and a crossover between them: In weak fields, tripron-antitripron pair creation occurs through the tunneling mechanism, and in strong fields, the system is described by an effective Hamiltonian breaking the protecting symmetries. We analyze the numerically obtained results in terms of the Dykhne--Davis--Pechukas theory and Floquet theory, verifying the universal picture of the many-body Keldysh crossover in the DC-driven quantum spin systems.

cond-mat.str-el↗

arXiv2024

Many-body effects on high-harmonic generation in Hubbard ladders

We show how many-body effects associated with background spin dynamics control the high-harmonic generation (HHG) in Mott insulators by analyzing the two-leg ladder Hubbard model. Spin dynamics activated by the interchain hopping $t_y$ drastically modifies the HHG features. When two chains are decoupled ($t_y=0$), HHG originates from the dynamics of coherent doublon-holon pairs because of spin-charge separation. With increasing $t_y$, the doublon-holon pairs lose their coherence due to their interchain hopping and resultant spin-strings. Furthermore, the HHG signal from spin-polarons -- charges dressed by spin clouds -- leads to an additional plateau in the HHG spectrum. For large $t_y$, we identify unconventional HHG processes involving $three$ elementary excitations -- two polarons and one magnon. Our results demonstrate the nontrivial nature of HHG in strongly correlated systems, and its qualitative differences to conventional semiconductors.

cond-mat.str-el↗

arXiv2023

Spin, charge and $η$-spin separation in one-dimensional photo-doped Mott insulators

We show that effectively cold metastable states in one-dimensional photo-doped Mott insulators described by the extended Hubbard model exhibit spin, charge and $η$-spin separation. Namely, their wave functions in the large on-site Coulomb interaction limit can be expressed as $|Ψ\rangle =|Ψ_{\rm charge}\rangle|Ψ_{\rm spin}\rangle |Ψ_{\rm η-spin}\rangle$, which is analogous to the Ogata-Shiba states of the doped Hubbard model in equilibrium. Here $η$-spin represents the type of the photo-generated pseudeparticles (doublon or holon). $|Ψ_{\rm charge}\rangle$ is determined by spinless free fermions, $|Ψ_{\rm spin}\rangle$ by the isotropic Heisenberg model in the squeezed spin space, and $|Ψ_{\rm η-spin}\rangle$ by the XXZ model in the squeezed $η$-spin space. In particular, the metastable $η$-pairing and charge-density-wave (CDW) states correspond to the gapless and gapful states of the XXZ model. The specific form of the wave function allows us to accurately determine the exponents of correlation functions. The form also suggests that the central charge of the $η$-pairing state is 3 and that of the CDW phase is 2, which we numerically confirm. Our study provides analytic and intuitive insights into the correlations between active degrees of freedom in photo-doped strongly correlated systems.

cond-mat.str-el↗

arXiv2023

Phase transitions and spin dynamics of the quasi-one dimensional Ising-like antiferromagnet BaCo$_{2}$V$_{2}$O$_{8}$ in a longitudinal magnetic field

By combining inelastic neutron scattering and numerical simulations, we study the quasi-one dimensional Ising-like quantum antiferromagnet BaCo$_{2}$V$_{2}$O$_{8}$ in a longitudinal magnetic field applied along the magnetic anisotropy axis, which is also the chain direction. The external field closes the excitation gap due to the magnetic anisotropy, inducing a transition from the Néel ordered state to an incommensurate longitudinal spin density wave phase. If the field is increased further, another transition into a transverse antiferromagnetic phase takes place at 9 T due to the competition between longitudinal and transverse correlations. We numerically and experimentally show that the model of XXZ chains connected by a weak interchain interaction well reproduces this transition. We also calculate the dynamical susceptibility and demonstrate that it agrees quantitatively with inelastic neutron scattering measurements. In contrast to the abrupt change of magnetic ordering, the spectra do not change much at the transition at 9 T, and the spin dynamics can be described as a Tomonaga-Luttinger liquid. We also refine the modeling of BaCo$_{2}$V$_{2}$O$_{8}$ by including a four-site periodic term arising from the crystal structure which enables to account for an anomaly of the magnetic susceptibility appearing at 19 T as well as for the anticrossing observed in the inelastic neutron scattering spectra.

cond-mat.str-el↗

arXiv2022

Dynamical conductivity of disordered quantum chains

We study the transport properties of a one dimensional quantum system with disorder. We numerically compute the frequency dependence of the conductivity of a fermionic chain with nearest neighbor interaction and a random chemical potential by using the Chebyshev matrix product state (CheMPS) method. As a benchmark, we investigate the noninteracting case first. Comparison with exact diagonalization and analytical solutions demonstrates that the results of CheMPS are reliable over a wide range of frequencies. We then calculate the dynamical conductivity spectra of the interacting system for various values of the interaction and disorder strengths. In the high frequency regime, the conductivity decays as a power law, with an interaction dependent exponent. This behavior is qualitatively consistent with the bosonized field theory predictions, although the numerical evaluation of the exponent shows deviations from the analytically expected values. We also compute the characteristic pinning frequency at which a peak in the conductivity appears. We confirm that it is directly related to the inverse of the localization length, even in the interacting case. We demonstrate that the localization length follows a power law of the disorder strength with an exponent dependent on the interaction, and find good quantitative agreement with the field theory predictions. In the low frequency regime, we find a behavior consistent with the one of the noninteracting system $ω^{2}(\lnω)^{2}$ independently of the interaction. We discuss the consequences of our finding for experiments in cold atomic gases.

cond-mat.quant-gas↗

arXiv2022

Second-order magnetic responses in quantum magnets: Magnetization under ac magnetic fields

We investigate second-order magnetic responses of quantum magnets against ac magnetic fields. We focus on the case where the $z$ component of the spin is conserved in the unperturbed Hamiltonian and the driving field is applied in the $xy$ plane. We find that linearly polarized driving fields induce a second-harmonic response, while circularly polarized fields generate only a zero-frequency response, leading to a magnetization with a direction determined by the helicity. Employing an unbiased numerical method, we demonstrate the nonlinear magnetic effect driven by the circularly polarized field in the XXZ model and show that the magnitude of the magnetization can be predicted by the dynamical spin structure factor in the linear response regime.

cond-mat.str-el↗

arXiv2021

Exploring nonequilibrium phases of photo-doped Mott insulators with Generalized Gibbs ensembles

Photo-excited strongly correlated systems can exhibit intriguing non-thermal phases, but the theoretical investigation of them poses significant challenges. In this work, we introduce a generalized Gibbs ensemble type description for long-lived photo-doped states in Mott insulators. This framework enables systematic studies of photo-induced phases based on equilibrium methods, as demonstrated here for the one-dimensional extended Hubbard model. We determine the nonequilibrium phase diagram, which features $η$-pairing and charge density wave phases in a wide doping range, and reveal physical properties of these phases. We show that the peculiar kinematics of photo-doped carriers, and the interaction between them, play an essential role in the formation of the non-thermal phases, and we clarify the differences between photo-doped Mott insulators, chemically-doped Mott insulators and photo-doped semiconductors. Our results demonstrate a new path for the systematic exploration of nonequilibrium strongly correlated systems and show that photo-doped Mott insulators host different phases than conventional semiconductors.

cond-mat.str-el↗

arXiv2021

Nonadiabatic Nonlinear Optics and Quantum Geometry -- Application to the Twisted Schwinger Effect

We study the tunneling mechanism of nonlinear optical processes in solids induced by strong coherent laser fields. The theory is based on an extension of the Landau-Zener model with nonadiabatic geometric effects. In addition to the rectification effect known previously, we find two effects, namely perfect tunneling and counterdiabaticity at fast sweep speed. We apply this theory to the twisted Schwinger effect, i.e., nonadiabatic pair production of particles by rotating electric fields, and find a nonperturbative generation mechanism of the opto-valley polarization and photo-current in Dirac and Weyl fermions.

cond-mat.mes-hall↗

arXiv2021

Solitonic excitations in the Ising anisotropic chain BaCo2V2O8 under large transverse magnetic field

We study the dynamics of the quasi-one-dimensional Ising-Heisenberg antiferromagnet BaCo2V2O8 under a transverse magnetic field. Combining inelastic neutron scattering experiments and theoretical analyses by field theories and numerical simulations, we mainly elucidate the structure of the spin excitation spectrum in the high field phase, appearing above the quantum phase transition point mu0Hc ~ 10 T. We find that it is characterized by collective solitonic excitations superimposed on a continuum. These solitons are strongly bound in pairs due to the effective staggered field induced by the nondiagonal g tensor of the compound, and are topologically different from the fractionalized spinons in the weak field region. The dynamical susceptibility numerically calculated with the infinite time-evolving block decimation method shows an excellent agreement with the measured spectra, which enables us to identify the dispersion branches with elementary excitations. The lowest energy dispersion has an incommensurate nature and has a local minimum at an irrational wave number due to the applied transverse field.

cond-mat.str-el↗

arXiv2021

Mott Memristors based on Field-Induced Carrier Avalanche Multiplication

We present a theory of Mott memristors whose working principle is the non-linear carrier avalanche multiplication in Mott insulators subject to strong electric fields. The internal state of the memristor, which determines its resistance, is encoded in the density of doublon and hole excitations in the Mott insulator. In the current-voltage characteristic, insulating and conducting states are separated by a negative-differential-resistance region, leading to hysteretic behavior. Under oscillating voltage, the response of a voltage-controlled, non-polar memristive system is obtained, with retarded current and pinched hysteresis loop. As a first step towards neuromorphic applications, we demonstrate self-sustained spiking oscillations in a circuit with a parallel capacitor. Being based on electronic excitations only, this memristor is up to several orders of magnitude faster than previous proposals relying on Joule heating or ionic drift.

cond-mat.str-el↗

arXiv2020

High-harmonic generation in one-dimensional Mott insulator

We study high-harmonic generation (HHG) in the one-dimensional Hubbard model in order to understand its relation to elementary excitations as well as the similarities and differences to semiconductors. The simulations are based on the infinite time-evolving block decimation (iTEBD) method and exact diagonalization. We clarify that the HHG originates from the doublon-holon recombination, and the scaling of the cutoff frequency is consistent with a linear dependence on the external field. We demonstrate that the subcycle features of the HHG can be reasonably described by a phenomenological three step model for a doublon-holon pair. We argue that the HHG in the one-dimensional Mott insulator is closely related to the dispersion of the doublon-holon pair with respect to its relative momentum, which is not necessarily captured by the single-particle spectrum due to the many-body nature of the elementary excitations. For the comparison to semiconductors, we introduce effective models obtained from the Schrieffer-Wolff transformation, i.e. a strong-coupling expansion, which allows us to disentangle the different processes involved in the Hubbard model: intraband dynamics of doublons and holons, interband dipole excitations, and spin exchanges. These demonstrate the formal similarity of the Mott system to the semiconductor models in the dipole gauge, and reveal that the spin dynamics, which does not directly affect the charge dynamics, can reduce the HHG intensity. We also show that the long-range component of the intraband dipole moment has a substantial effect on the HHG intensity, while the correlated hopping terms for the doublons and holons essentially determine the shape of the HHG spectrum. A new numerical method to evaluate single-particle spectra within the iTEBD method is also introduced.

cond-mat.str-el↗

arXiv2020

Diagrammatic study of optical excitations in correlated systems

The optical conductivity contains relevant information on the properties of correlated electron systems. In infinite dimensions, where dynamical mean field theory becomes exact, vertex corrections can be neglected and the conductivity computed from particle-hole bubbles. An interesting question concerns the nature and effect of the most relevant vertex corrections in finite-dimensional systems. A recent numerical study showed that the dominant vertex correction near an ordering instability with wave vector π comes from a vertical ladder, analogous to the Maki-Thompson diagram. Since the RPA version of this ladder diagram, dubbed π-ton, can be easily evaluated, this suggests a simple procedure for incorporating antiferromagnetic or charge density wave fluctuations into dynamical mean field estimates of the optical conductivity and related susceptibilities. We implement this procedure for the half-filled Hubbard model, considering the π-ton and a double-ladder extension of the π-ton, and reveal the spectral signatures of these vertex corrections.

cond-mat.str-el↗

arXiv2020

Optomagnonic Barnett effect

Combining the technologies of quantum optics and magnonics, we find that the circularly polarized laser can dynamically realize the quasiequilibrium magnon Bose-Einstein condensates (BEC). The Zeeman coupling between the laser and spins generates the optical Barnett field, and its direction is controllable by switching the laser chirality. We show that the optical Barnett field develops the total magnetization in insulating ferrimagnets with reversing the local magnetization, which leads to the quasiequilibrium magnon BEC. This laser-induced magnon BEC transition through optical Barnett effect, dubbed the optomagnonic Barnett effect, provides an access to coherent magnons in the high frequency regime of the order of terahertz. We also propose a realistic experimental setup to observe the optomagnonic Barnett effect using current device and measurement technologies as well as the laser chirping. The optomagnonic Barnett effect is a key ingredient for the application to ultrafast spin transport.

cond-mat.mes-hall↗

arXiv2020

STM as a single Majorana detector of Kitaev's chiral spin liquid

In this letter, we propose a local detection scheme for the Majorana zero mode (MZM) carried by a vison in Kitaev's chiral spin liquid (CSL) using scanning tunneling microscopy (STM). The STM introduces a single Majorana into the system through hole/charge injection and the Majorana interacts with the MZM to form a stable composite object. We derive the exact analytical expression of single-hole Green's function in the Mott insulating limit of Kitaev's model and show that the differential conductance has split peaks, as a consequence of resonant tunneling through the vison-hole composite. The peak splitting scales with the binding energy of vison-hole composite, which is comparable to the Majorana gap in CSL, well within the reach of experimental observation.

cond-mat.str-el↗