arXiv subjects

Masahiro Sato

Publications and source records attributed to Masahiro Sato.

At least 19 recordsLinked to original sources

arXiv2026

Giant staggered Dzyaloshinskii-Moriya vectors emerged in synthetic antiferromagnets with spatially alternating stress

Dzyaloshinskii-Moriya (DM) interaction is a source of chiral magnetic physics, and it manifests microstructural symmetry which could link to exotic magnetic and electronic properties such as altermagnetism. For a new paradigm in magnetism and spintronics, it is crucial to have control on the DM interaction as well as ordering of the DM vectors which characterize the DM interaction. Here, we report on a strong interlayer DM interaction emerged in Co/Ru magnetic superlattices whose structural symmetry is unambiguously broken by alternating mechanical strains developed using the peculiar technique of our stress mechanism. Moreover, theoretical analyses reveal that our magnetic superlattices host a staggered order of the DM vectors. Our results indicate that the spatially varying strains with our breakthrough technique are quite effective in breaking the symmetry of the system and prove that it can control the DM interaction strength as well as the DM vector order.

cond-mat.mtrl-sci↗

arXiv2026

Floquet Theory for Light-Driven Rotation of Dipolar and Multipolar Particles

Nano- or micro-particle rotation driven by light has been well known in the fields of optical manipulation and optical physics since the end of the last century. It is viewed as a sort of angular-momentum transfer from light to material, but its microscopic analysis based on the Hamiltonian or the equation of motion has been less developed. We model this rotation with a simple setup of an electrically dipolar or multipolar particle irradiated by circularly polarized laser and comprehensively analyze the Langevin-type equation of motion by using the Floquet theory for dissipative classical systems and the mode separation method. Furthermore, we numerically compute the time evolution of the particle. As a result, we accurately estimate the dependence of the laser-frequency, laser-intensity, particle mass, temperature, and friction (dissipation) on the laser-driven rotation. We determine the ``nonequilibrium phase diagram'' of the laser-driven rotation in a broad parameter regime, which consists of three regimes: the rotation frequency $Ω\proptoω^{-1}$, $Ω\proptoω^{-3}$, or $Ω=ω$ ($ω$ is the laser frequency). Comparing our theoretical result with some experiments, we show that the result of the overdamped Langevin equation is qualitatively consistent with the experiments.

cond-mat.mes-hall↗

arXiv2026

Magnon harmonic generation in antiferromagnets: Dynamical symmetry enriched by symmetry breaking

In recent years, techniques of intense THz laser have enabled us to experimentally observe nonlinear spin dynamics in antiferromagnets since the elementary excitations such as magnons reside on a THz to GHz range in antiferromagnets and THz laser thus can directly excite them. We numerically and theoretically investigate THz-laser or GHz-wave driven harmonic generations in typical ordered phases of antiferromagnets: Néel, canted and weak ferromagnetic phases. The radiation waves (harmonic generations) are created by the incident-wave driven magnon dynamics. We point out that magnetic orders and phase transitions can change the spectra of harmonic generations, differently from those of metallic, semiconductor, or atomic-gas systems without (spontaneous) symmetry breakings. We consider both the magnon harmonic generation driven by standard single-color laser and that by two-color laser in the antiferromagnets, and find several dynamical symmetries and the corresponding selection rules of the harmonic generations. These results indicate that the magnon harmonic generation spectra provide new information about symmetry or symmetry breaking of antiferromagnets.

cond-mat.str-el↗

arXiv2026

GAI: Generative Agents for Innovation

This study examines whether collective reasoning among generative agents can facilitate novel and coherent thinking that leads to innovation. To achieve this, it proposes GAI, a new LLM-empowered framework designed for reflection and interaction among multiple generative agents to replicate the process of innovation. The core of the GAI framework lies in an architecture that dynamically processes the internal states of agents and a dialogue scheme specifically tailored to facilitate analogy-driven innovation. The framework's functionality is evaluated using Dyson's invention of the bladeless fan as a case study, assessing the extent to which the core ideas of the innovation can be replicated through a set of fictional technical documents. The experimental results demonstrate that models with internal states significantly outperformed those without, achieving higher average scores and lower variance. Notably, the model with five heterogeneous agents equipped with internal states successfully replicated the key ideas underlying the Dyson's invention. This indicates that the internal state enables agents to refine their ideas, resulting in the construction and sharing of more coherent and comprehensive concepts.

cs.AI↗

arXiv2026

Inverse Chiral Phonon Zeeman Effect in Noncentrosymmetric Crystals

We present a microscopic theory of the inverse chiral phonon Zeeman effect in noncentrosymmetric crystals. Within micropolar elasticity, coupled translational displacements and microrotations give rise to intrinsically chiral phonons, which generate an elliptically polarized internal magnetic field through dynamical piezoelectricity. In the high-frequency Floquet regime and under incomplete electronic screening, this field acts as an effective longitudinal Zeeman field on electronic spins, leading to spin polarization and band splitting. The results establish a purely lattice-driven mechanism for the inverse chiral phonon Zeeman effect in noncentrosymmetric crystals.

cond-mat.mtrl-sci↗

arXiv2025

Optical-vortex-pulse induced nonequilibrium spin textures in spin-orbit coupled electrons

Optical vortex beams are a type of topological light characterized by their inherent orbital angular momentum, leading to the propagation of a spiral-shaped wavefront. In this study, we focus on two-dimensional electrons with Rashba and Dresselhaus spin-orbit interactions and examine how they respond to pulsed vortex beams in the terahertz frequency band. Spin-orbital interactions play a vital role in transferring the orbital angular momentum of light to electron systems and generating spatiotemporal spin textures. We show that the spatiotemporal spin polarization of electrons reflects orbital angular momentum carried by optical vortex pulses. These findings demonstrate how optical vortices facilitate ultrafast spin manipulation in spin-orbit-coupled electrons. Our results can be straightforwardly extended to the case of higher-frequency vortex beams for other two-dimensional metals with a larger Fermi energy.

cond-mat.mes-hall↗

arXiv2025

Floquet theory and applications in open quantum and classical systems

This article reviews theoretical methods for analyzing Floquet engineering (FE) phenomena in open (dissipative) quantum or classical systems, with an emphasis on our recent results. In many theoretical studies for FE in quantum systems, researchers have used the Floquet theory for closed (isolated) quantum systems, that is based on the Schrödinger equation. However, if we consider the FE in materials driven by an oscillating field like a laser, a weak but finite interaction between a target system and an environment (bath) is inevitable. In this article, we describe these periodically driven dissipative systems by means of the quantum master (GKSL) equation. In particular, we show that a nonequilibrium steady state appears after a long driving due to the balance between the energy injection by the driving field and the release to the bath. In addition to quantum systems, if we try to simply apply Floquet theory to periodically driven classical systems, it failed because the equation of motion (EOM) is generally nonlinear, and the Floquet theorem can be applied only to linear differential equations. Instead, by considering the distribution function of the classical variables (i.e., Fokker-Planck equation), one can arrive at the effective EOM for the driven systems. We illustrate the essence of the Floquet theory for classical systems. On top of fundamentals of the Floquet theory, we review representative examples of FEs (Floquet topological insulators, inverse Faraday effects in metals and magnets, Kapitza pendulum, etc.) and dissipation-assisted FEs.

cond-mat.str-el↗

arXiv2025

Two-color laser control of photocurrent and high harmonics in graphene

We comprehensively investigate two-color-laser-driven photocurrent and high harmonic generation (HHG) in graphene models. By numerically solving the quantum master equation, we uniformly explore a broad parameter regime including both the weak (perturbative) and intense-laser (nonperturbative) cases while considering the dissipation effects. We demonstrate that the HHG spectra can be drastically altered by tuning the real-space path traced by the laser electric field. This controllability is explained by the dynamical symmetry argument. We also show that both the magnitude and the direction of photocurrent (zeroth order harmonics) can be controlled by varying the frequency, intensity, ellipticity, and relative phase of the two-color laser. Furthermore, the nature of photocurrent is shown to be classified into shift- or injection-current types, depending on the phase of two-color laser. Our findings indicate that even in centrosymmetric electron systems, photocurrent and HHG can be quantitatively controlled by adjusting various external parameters if we utilize multicolor laser with a lower spatial or temporal symmetry.

cond-mat.mes-hall↗

arXiv2025

Generation of magnetic chiral solitons, skyrmions, and hedgehogs with electric fields

Electric-field controls of Dzyaloshinskii-Moriya interactions (DMIs) have recently been discussed from the microscopic viewpoint. Since the DMI plays a critical role in generating topological spin textures (TSTs) such as the chiral soliton, the magnetic skyrmion, and the magnetic hedgehog, electric-field controls of these TSTs have become an important issue. This paper shows that such electric-field-induced DMI indeed creates and annihilates TSTs by numerically solving the Landau-Lifshitz-Gilbert (LLG) equation for many-body spin systems at finite temperatures. We show that when a strong electric field is applied in a proper way to one- or two-dimensional ferromagnets, the Hamiltonians are changed into the well-known spin models for the chiral soliton or the skyrmion lattice, and the TST states emerge. We utilize a machine-learning method to count the number of generated TSTs. In the three-dimensional (3D) case, we demonstrate the electric-field induction of a magnetic hedgehog structure as follows: Applying a strong enough electric field along a proper direction to a skyrmion-string state (a triple-$\boldsymbol{q}$ state) at low but finite temperatures, we find that the field-induced DMI can drive a quadruple-$\boldsymbol{q}$ state with hedgehog-antihedgehog pairs. This result indicates that we have succeeded in constructing a simple 3D short-range interacting spin model hosting a magnetic hedgehog structure.

cond-mat.str-el↗

arXiv2025

Nonlinear optical response of truly chiral phonons: Light-induced phonon angular momentum, Peltier effect, and orbital current

The nonlinear optical responses of chiral phonons to terahertz and infrared light are studied using the nonlinear response theory. We show that the photo-induced angular momentum increases with the square of the chiral-phonon relaxation time $τ$, giving a significantly larger angular momentum compared to ordinary phonons. We also find that the photo-induced Peltier effect by chiral phonons occurs through a mechanism distinct from those proposed recently; the induced energy current scales $\proptoτ^2$, giving a larger energy current in the clean limit. We prove a linear relation between the generated angular momentum and the energy current. Lastly, we show that the orbital current, an analog of the spin current, occurs through a nonlinear response. These findings demonstrate the unique properties and functionalities of chiral phonons.

cond-mat.mes-hall↗

arXiv2025

Signature of BKT-like spin transport in a quasi-2D antiferromagnet BaNi$_2$V$_2$O$_8$

In two-dimensional (2D) spin systems, the augmentation of spin fluctuations gives rise to quasi-long-range order; however, how they manifest in spin transport remains unclear. Here we investigate the spin Seebeck effect (SSE) in a quasi-2D antiferromagnet, BaNi$_2$V$_2$O$_8$, which has been reported to exhibit the Berezinskii-Kosterlitz-Thouless (BKT) transition owing to its distinct 2D nature. We found that the SSE in Pt / BaNi$_2$V$_2$O$_8$ persists well above the Néel temperature, significantly different from the behavior of 3D ordered magnets. Our numerical analysis for a 2D microscopic spin model supports the hypothesis that the observed SSE is linked to strong magnetic correlations in the BKT-like phase.

cond-mat.str-el↗

arXiv2025

Spin Seebeck Effect as a Probe for Majorana Fermions in Kitaev Spin Liquids

Quantum entanglement in strongly correlated electron systems often leads to exotic elementary excitations. Quantum spin liquids (QSLs) provide a paradigmatic example, where the elementary excitations are described by fractional quasiparticles such as spinons. However, such fractional quasiparticles behave differently from electrons, making their experimental identification challenging. Here, we theoretically investigate the spin Seebeck effect, which is a thermoelectric response via a spin current, as an efficient probe of the fractional quasiparticles in QSLs, focusing on the Kitaev honeycomb model. By comprehensive studies using the real-time dynamics, the perturbation theory, and the linear spin-wave theory based on the tunnel spin-current theory, we find that the spin current is induced by thermal gradient in the Kitaev spin liquid, via the low-energy fractional Majorana excitations. This underscores the ability of Majorana fermions to carry spin current, despite lacking spin angular momentum. Furthermore, we find that the induced spin current changes its sign depending on the sign of the Kitaev interaction, indicating that the Majorana fermions contribute to the spin current with (up-)down-spin like nature when the exchange coupling is (anti)ferromagnetic. Thus, in contrast to the negative spin current already found in a one-dimensional QSL, our finding reveals that the spin Seebeck effect can exhibit either positive or negative signals, contingent upon the nature of fractional excitations in the QSLs. We also clarify contrasting field-angle dependence between the Kitaev spin liquid in the low-field limit and the high-field ferromagnetic state, which is useful for the experimental identification. Our finding suggests that the spin Seebeck effect could be used not only to detect fractional quasiparticles emerging in QSLs but also to generate and control them.

cond-mat.str-el↗

arXiv2024

Generalized Back-Stepping Experience Replay in Sparse-Reward Environments

Back-stepping experience replay (BER) is a reinforcement learning technique that can accelerate learning efficiency in reversible environments. BER trains an agent with generated back-stepping transitions of collected experiences and normal forward transitions. However, the original algorithm is designed for a dense-reward environment that does not require complex exploration, limiting the BER technique to demonstrate its full potential. Herein, we propose an enhanced version of BER called Generalized BER (GBER), which extends the original algorithm to sparse-reward environments, particularly those with complex structures that require the agent to explore. GBER improves the performance of BER by introducing relabeling mechanism and applying diverse sampling strategies. We evaluate our modified version, which is based on a goal-conditioned deep deterministic policy gradient offline learning algorithm, across various maze navigation environments. The experimental results indicate that the GBER algorithm can significantly boost the performance and stability of the baseline algorithm in various sparse-reward environments, especially those with highly structural symmetricity.

cs.LG↗

arXiv2024

Floquet Theory and Ultrafast Control of Magnetism

The development of laser science and technology have stimulated the study of condensed matter physics, especially, dynamical or non-equilibrium nature in solids. The laser technique in terahertz (THz) regime, whose photon energy is comparable to those of typical collective modes in solids such as magnetic excitations, phonons, etc., has remarkably proceeded in the last decade. Theoretical tools for non-equilibrium states have also progressed. Thanks to these backgrounds, magneto-optics, especially, the study of controlling magnetism with laser, now enters a new stage. For such controls, Floquet engineering is a key concept, which means the method of controlling static properties of targets with high-frequency external fields like laser. I review the theoretical foundation of Floquet engineering and its application to magnetic insulators. Basic magnetic quantities such as magnetization, spin chirality, and spin current are shown to be controlled with intense THz laser or wave.

cond-mat.str-el↗

arXiv2024

Triplons, triplon pairs and dynamical symmetries in laser-driven Shastry-Sutherland magnets

In frustrated magnets, a variety of novel phenomena arise due to their peculiar magnetic states, whose excitations usually reside in the GHz to THz range. Intense THz lasers, therefore, lead to examining nonlinear optical effects in such magnets. Employing an unbiased numerical method, we calculate the laser-pulse driven harmonic spectra of the Shastry-Sutherland magnets, which is well known as the model of a quantum frustrated magnet $SrCu_2(BO_3)_2$. As a result, nonlinear responses of triplons and a two-triplon bound state are observed through the Zeeman or magnetoelectric couplings between the laser and the electron spin. Furthermore, we find that one can obtain information on magnetic anisotropy and symmetry from the spectra by manipulating external fields or two-color lasers. Our results indicate that laser-driven harmonic spectra are useful as a probe of less-detectable quantum many-body states in frustrated magnets.

cond-mat.str-el↗

arXiv2024

Calibrating the Predictions for Top-N Recommendations

Well-calibrated predictions of user preferences are essential for many applications. Since recommender systems typically select the top-N items for users, calibration for those top-N items, rather than for all items, is important. We show that previous calibration methods result in miscalibrated predictions for the top-N items, despite their excellent calibration performance when evaluated on all items. In this work, we address the miscalibration in the top-N recommended items. We first define evaluation metrics for this objective and then propose a generic method to optimize calibration models focusing on the top-N items. It groups the top-N items by their ranks and optimizes distinct calibration models for each group with rank-dependent training weights. We verify the effectiveness of the proposed method for both explicit and implicit feedback datasets, using diverse classes of recommender models.

cs.IR↗

arXiv2024

High-harmonic generation in graphene under the application of a DC electric current: From perturbative to nonperturbative regimes

We theoretically investigate high-harmonic generation (HHG) in honeycomb-lattice graphene models when subjected to a DC electric field. By integrating the quantum master equation with the Boltzmann equation, we develop a numerical method to compute laser-driven dynamics in many-electron lattice systems under DC electric current. The method enables us to treat both the weak-laser (perturbative) and intense-laser (nonperturbative) regimes in a unified way, accounting for the experimentally inevitable dissipation effects. From it, we obtain the HHG spectra and analyze their dependence on laser frequency, laser intensity, laser-field direction, and DC current strength. We show that the dynamical and static symmetries are partially broken by a DC current or staggered potential term, and such symmetry breakings drastically change the shape of the HHG spectra, especially in terms of the presence or absence of $(2n+1)$th-, $2n$th-, or $3n$th-order harmonics ($n\in \mathbb Z$). The laser intensity, frequency, and polarization are also shown to affect the shape of the HHG spectra. Our findings indicate that HHG spectra in conducting electron systems can be quantitatively or qualitatively controlled by tuning various external parameters, and DC electric current is used as such an efficient parameter.

cond-mat.mes-hall↗

arXiv2024

Theory of the inverse Faraday effect in dissipative Rashba electron systems: Floquet engineering perspective

We theoretically study the inverse Faraday effect (IFE), i.e., photo-induced magnetization, in two-dimensional Rashba spin-orbit coupled electron systems irradiated by a circularly polarized light. The quantum master (Gorini-Kossakowski-Sudarshan-Lindblad) equation enables us to accurately compute the laser driven dynamics, taking inevitable dissipation effects into account. To find the universal features of laser-driven magnetization and its dynamics, we comprehensively investigate (i) the nonequilibrium steady state (NESS) driven by a continuous wave and (ii) ultrafast spin dynamics driven by short laser pulses. In the NESS (i), the laser-induced magnetization and its dependence of several parameters (laser frequency, laser field strength, temperature, dissipation strength, etc.) are shown to be in good agreement with the predictions from Floquet theory for dissipative systems in the high-frequency regime. In the case (ii), we focus on ferromagnetic metal states by introducing an effective magnetic field to the Rashba model as the mean field of electron-electron interaction. We find that a precession of the magnetic moment occurs due to the pulse-driven instantaneous magnetic field and the initial phase of the precession is controlled by changing the sign of light polarization. This is well consistent with the spin dynamics observed in experiments of laser-pulse-driven IFE. We discuss how the pulse-driven dynamics are captured by the Floquet theory. Our results %pave the way for computing provides a microscopic method to compute ultrafast dynamics in many electron systems irradiated by intense light.

cond-mat.mes-hall↗