arXiv subjects

Satoru Hayami

Publications and source records attributed to Satoru Hayami.

At least 37 records · Page 2Linked to original sources

arXiv2026

Shape dependence of Edelstein and magnetoelectric effects in the V-shaped model

We theoretically investigate the shape dependence and microscopic mechanism of the magnetoelectric (ME) effect, including both nonmagnetic (Edelstein-type) and magnetic origins, in a V-shaped one-dimensional chain model. Our goal is to establish a symmetry-based framework linking local geometry to ME responses. Numerical calculations based on the Kubo formula reveal that the nonmagnetic-driven ME response is maximized at an apex angle of $θ\approx 0.6π$. To clarify its origin, we derive a low-energy effective Hamiltonian in the $s$-orbital subspace and demonstrate that the polarity induced by the V-shaped geometry manifests as an effective spin--orbit interaction. An analytical derivation of the Green's function shows that the geometric effect can be described as a $T$-matrix contribution associated with local symmetry breaking. This formulation provides a unified description of geometry-induced responses in terms of a scattering framework. Using a multipole-basis representation, we identify symmetry-based selection rules for the ME tensor and show that the coupling between the effective spin--orbit interaction and the orbital angular momentum generated across the apex plays an essential role. The resulting angular dependence, $\sinθ\sin{θ/2}$, peaks at $θ= 2\tan^{-1}\sqrt{2} \approx 0.608π$, in good agreement with the numerical results. We also analyze a ferromagnetic V-shaped model including the Zeeman interaction and show that the magnetic-driven ME response originates from the spin magnetization induced by the coupling between the electric-field--driven charge-potential gradient and the Zeeman term. These results reveal distinct ME mechanisms depending on the presence or absence of time-reversal symmetry and provide a microscopic framework for geometry-induced multipole phenomena.

cond-mat.mes-hall↗

arXiv2026

Effects of crystal field and momentum-based frustrated exchange interactions on multiorbital square skyrmion lattice

Motivated by recent theoretical predictions of a square-shaped skyrmion lattice (S-SkL) in centrosymmetric tetragonal Ce-based magnets [Yan Zha and Satoru Hayami, Phys. Rev. B 111, 165155 (2025)], we perform a comprehensive theoretical investigation into the role of multiorbital effects and momentum-based frustrated exchange interactions in stabilizing such topologically nontrivial magnetic textures. By employing self-consistent mean-field calculations over a broad range of model parameters, we demonstrate that the cooperative interplay among interorbital coupling, frustrated exchange interactions at higher-harmonic wave vectors, and crystal-field-induced anisotropy is crucial for the stabilization of the S-SkL. Furthermore, the competition between the easy-plane intraorbital anisotropy and the easy-axis interorbital anisotropy leads to a significant enhancement of the S-SkL stability region. We also identify a rich variety of multi-$Q$ states, including a topologically nontrivial S-SkL state with a slight breaking of fourfold rotational symmetry (S-SkL$'$), magnetic bubble lattices (MBLs), and double-$Q$ phases with a local/net scalar chirality. Our findings elucidate the microscopic mechanism responsible for the emergence of S-SkLs in prototypical Ce-based magnets and provide a route toward realizing skyrmion lattices in a broader class of $f$-electron materials beyond conventional Gd- and Eu-based systems lacking orbital angular momentum.

cond-mat.str-el↗

arXiv2026

Antisymmetric linear transverse magnetization and ferroaxial moments induced by geometry-driven electric field gradients

We theoretically investigate the transverse magnetization and ferroaxial moments induced by electric field gradients arising from the geometry of finite systems. Based on the Kubo formalism and real-time numerical simulations for a finite trapezoidal model, we demonstrate that both quantities are generated under the electric field gradient and are enhanced by tuning the leg inclination, which controls the gradient strength. We further show that the induced transverse magnetization is antisymmetric and linear in the magnetic field; such a response is prohibited by Onsager reciprocity in the absence of an electric field gradient. In addition, we find that the total transverse magnetization scales linearly with the electric field, in contrast to the longitudinal one, which exhibits a quadratic dependence, providing an advantage for experimental observation. Our results establish geometry-induced electric field gradients as a versatile mechanism for realizing and controlling unconventional transverse responses in mesoscopic systems.

cond-mat.mes-hall↗

arXiv2026

Light-induced nonlinear Edelstein effect under ferroaxial ordering

Ferroaxial ordering, a spontaneous rotational distortion of the atomic arrangement, brings about a cross-product-type spin-orbit coupling (SOC) manifested as an electric toroidal dipole. We propose the light-induced nonlinear Edelstein effect (NLEE) -- a second-order optical response in which a static magnetization is induced by a time-dependent electric field -- as a promising probe of ferroaxial ordering. First, we elucidate the relationship between the NLEE tensor and the electric toroidal dipole. By decomposing the polarization modes of light, we find that both the linearly polarized and circularly polarized light couple to the electric toroidal dipole via distinct mechanisms. We then demonstrate the NLEE using a minimal model that incorporates ferroaxial ordering. Our analysis reveals that effective coupling between orbital magnetization and SOC induces spin magnetization. In particular, the spin magnetization is tilted owing to the electric toroidal dipole; the tilt angle reflects the ratio between the ferroaxial-origin SOC and the relativistic SOC.

physics.optics↗

arXiv2026

Effective phonon models based on symmetry-adapted multipole basis -- Hidden chiral phonon angular momentum splitting in ferroaxial systems

We propose a symmetry-based framework for constructing effective harmonic phonon models using a symmetry-adapted multipole basis. By decomposing the force-constant matrix into bond-centered electric multipoles, we identify the minimal microscopic ingredients responsible for phonon angular-momentum splitting. Applying this framework to a minimal zigzag-chain model, we show that ferroaxial order gives rise to a hidden sublattice-resolved chiral phonons, while an additional polar contribution leads to finite global chirality. Our results provide a unified symmetry-based description of hidden and emergent phonon phenomena and suggest a route to control phonon properties via electronic orderings and external fields.

cond-mat.mtrl-sci↗

arXiv2026

Skyrmion generation via Laguerre-Gaussian beam irradiation in frustrated magnets

Since its discovery, the study of magnetic skyrmions has been on the rise. In this paper, we discuss our investigations on the light-induced mechanisms for skyrmion generation in a centrosymmetric triangular magnetic lattice with competing $J_1$-$J_3$ interactions, and easy-axis anisotropy. We solve the stochastic Landau-Lifshitz-Gilbert equation for the lattice spin dynamics under Laguerre-Gaussian beam irradiation. Numerical results show that skyrmions are nucleated in two thermodynamic regions, each favoring different phases: the ferromagnetic phase and the skyrmion-lattice phase. In the ferromagnetic region, isolated skyrmions are generated mainly through stochastic thermal nucleation. In this regime, higher temperatures and larger beam widths are required to overcome the nucleation barrier. In contrast, in the skyrmion-lattice region, skyrmion nucleation occurs via thermal annealing, where the system relaxes toward its true ground state. These findings establish a comprehensive theoretical framework for optimizing optical control to generating light-induced skyrmionic textures in frustrated magnets.

cond-mat.str-el↗

arXiv2026

Antiparallel spin polarizations as quadratic response in chiral systems

Chirality-dependent spin generation has attracted considerable attention in condensed matter physics. In this paper, we theoretically investigate antiparallel spin polarization as a chirality-dependent quadratic response, by using a finite chiral system composed of triangular prisms. Based on the nonlinear Kubo formalism and real-time simulations, we demonstrate that spatially inhomogeneous antiparallel spin polarizations are induced as a dissipative quadratic DC response to a homogeneous AC electric field. In particular, we elucidate role of microscopic parameters characterizing the handedness of chirality, and naive expectation of spin polarization as a consequence of spin accumulation of spin current.

cond-mat.mes-hall↗

arXiv2026

Analysis of Spin Current Generation by Elastic Waves in $f$-wave Altermagnets

We theoretically investigate the mechanism of spin current generation induced by elastic waves in nonrelativistic magnets referred to as altermagnets. By analyzing an $f$-wave altermagnet formed by a three-sublattice noncollinear antiferromagnetic structure breaking the spatial inversion symmetry on a two-dimensional triangular lattice within the linear response theory, we show that the nonrelativistic antisymmetric spin-split band structure can give rise to spin current generation when either longitudinal or transverse elastic wave is applied. We find that the momentum dependence of the antisymmetric spin splitting leads to a characteristic direction-dependent spin current response. We also compare the present nonrelativistic magnetic-order-driven mechanism with the relativistic one in a nonmagnetic Rashba system. These findings highlight the potential of invesion-symmetry-breaking altermagnets as a spin current generator driven by elasticity without relying on the relativistic spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2026

Quantum theory of magnetic octupole in periodic crystals and application to $d$-wave altermagnets

Magnetic multipoles have been recognized as order parameters characterizing magnetic structure in solids. Recently, magnetic octupoles have been proposed as the order parameters of time-reversal-symmetry breaking centrosymmetric antiferromagnets exhibiting nonrelativistic spin splitting, which is referred to as ``altermagnet''. However, a gauge-invariant formulation of magnetic octupoles in crystalline solids remains elusive. Here, we present a gauge-invariant expression of spin magnetic octupoles in periodic crystals based on quantum mechanics and thermodynamics, which can be used to quantitatively characterize time-reversal-symmetry breaking antiferromagnets including $d$-wave altermagnets. The allowed physical response tensors are classified beyond symmetry considerations, and direct relationships are established for some of them in insulators at zero temperature. Furthermore, our expression reveals a contribution from an anisotropic magnetic dipole, which has the same symmetry as conventional spin and orbital magnetic dipoles but carries no net magnetization. We discuss the relation between the anisotropic magnetic dipole and the anomalous Hall effect.

cond-mat.mes-hall↗

arXiv2026

The 2026 Skyrmionics Roadmap

Magnetic skyrmions and related topological spin textures have emerged as a central topic in condensed-matter physics, combining fundamental significance with potential for transformative applications in spintronics, magnonics, and beyond. Over the past decade, advances in material platforms, imaging techniques, theoretical modeling, and device concepts have established skyrmionics as a rapidly expanding field. At the same time, challenges remain in stabilizing, controlling, and integrating such textures into functional architectures, while novel phenomena such as antiskyrmions, higher-order skyrmions, hopfions, and antiferromagnetic textures arise. The 2026 Skyrmionics Roadmap represents a collective effort of many authors, providing a comprehensive perspective on the current state-of-the-art and the outlook for the coming years. In 33 focused sections, each co-authored by two researchers, we chart progress in theory and modeling, material systems, skyrmion dynamics, and skyrmion technologies. By offering a consolidated vision, this Roadmap aims to guide both fundamental research and application-driven efforts, accelerating the transition of skyrmionics from conceptual breakthroughs toward practical technologies.

cond-mat.mes-hall↗

arXiv2026

Nonrelativistic Functional Properties in Collinear Antiferromagnets Based on Multipole Representation Theory

In recent years, the concept of multipoles has been widely used to describe and classify various magnetic and electric responses in solids, providing a systematic way to identify symmetry-allowed or -forbidden physical responses. Conventionally, multipole classifications rely on the magnetic point group of a system, which inherently incorporates the effects of relativistic spin-orbit coupling because the spin orientation is supposed to follow the point-group transformation of the lattice. However, this approach becomes insufficient in situations where relativistic spin-orbit coupling is negligibly weak or where the spin and orbital (lattice) degrees of freedom are decoupled, thereby requiring a more comprehensive symmetry description. In this work, we introduce a multipole description on the basis of the spin-point-group symmetries, enabling a systematic exploration of nonrelativistic phenomena that persist even without spin-orbit coupling in a collinear antiferromagnet. As an application, we theoretically demonstrate spin-current generation driven by elastic waves in a specific collinear antiferromagnet, fully independent of spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2026

Multiple-$Q$ spin textures induced by spiral--staggered interference in one-dimensional itinerant magnets

We theoretically investigate multiple-$Q$ magnetic states emerging from the interference between finite-$Q$ spiral and staggered spin modulations in a one-dimensional itinerant electron system. The multiple-$Q$ spin textures are characterized by a superposition of symmetry-unrelated ordering wave vectors in the same direction with distinct periodicities rather than rotationally symmetry-related ones. Motivated by recent experimental observations of broken helix magnetic structures in EuIn$_2$As$_2$, we focus on the microscopic interaction conditions in stabilizing such multiple-$Q$ states.We employ two effective spin models: One is the momentum-space-based model, and the other is the real-space-based model, both of which include bilinear and biquadratic easy-plane anisotropic interactions. By analyzing their ground state via simulated annealing, we find that a superposition of a spiral and a staggered modulation yields a robust double-$Q$ magnetic structure. Moreover, we demonstrate that the obtained double-$Q$ spin configuration exhibits an antisymmetric spin-split band structure even without the relativistic Dzyaloshinskii-Moriya interaction, and further reveals asymmetric band modulations when the magnetic field is applied along the out-of-plane direction. Our results provide a theoretical framework for understanding unconventional multiple-$Q$ magnetic textures in one-dimensional systems.

cond-mat.str-el↗

arXiv2025

Spinless electric toroidal multipoles in ferroaxial ${\rm K_2Zr(PO_4)_2}$ revealed by symmetry-adapted closest Wannier analysis

From a symmetry perspective, ferroaxial order belongs to the same symmetry as time-reversal-even pseudovectors. Experimentally, ${\rm K_2Zr(PO_4)_2}$ is known to undergo a displacive-type phase transition from a non-ferroaxial to a ferroaxial phase. To identify the key microscopic ingredients driving this transition, we carry out a quantitative analysis combining density-functional theory calculations and symmetry-adapted closest Wannier analysis. As a result, we show that electric toroidal dipole, electric toroidal octupole, and electric hexadecapole, which belong to the same irreducible representation, make dominant contributions to the ferroaxial transition. In particular, we find that spinless electric toroidal octupoles, which originate from spin-independent off-diagonal real hopping between the $p$ orbitals on P and O atoms and between the $d$ orbitals on Zr atoms and $p$ orbitals on O atoms, provide the most significant contributions. Moreover, we explicitly analyze the orbital characters involved in the relevant hybridizations associated with these multipoles. We further show that the relativistic spin--orbit coupling has a negligible influence on the ferroaxial transition. These results demonstrate that spin-independent orbital hybridization between different orbitals on different atoms plays a crucial role in inducing the ferroaxial transition.

cond-mat.mtrl-sci↗

arXiv2025

Cascade of Spin Moiré Superlattices with In-Plane Field in Triangle Lattice Semimetal EuAg$_4$Sb$_2$

EuAg$_4$Sb$_2$ is a rhombohedral europium triangle lattice material that exhibits a rich phase diagram of spin moiré superlattices (SMS) and single-$q$ magnetic phases. In this paper, we characterize the incommensurate phases accessible with field applied in the plane with small angle neutron scattering (SANS). A variety of phases with unusual SANS patterns are accessible with magnetic field applied along the $a$ and $a^*$ directions. Many of these phases can be understood to be multi-$q$ phases. One phase in particular, ICM2b (ICM=incommensurate magnetic phase), is rather unconventional in that it is an anisotropic multi-$q$ phase that can rotate freely within the $ab$-plane, dependent on magnetic field direction and history. The stabilization of tunable multi-$q$ incommensurate spin textures \textit{via} in-plane field sets this class of materials apart from conventional skyrmion materials. We further identify that the propagation vectors of the in-plane phases have a significant commensuration with the diameter of the smallest pocket of the Fermi surface ($2k_{\text{F}}$). The multi/single-$q$ nature is also correlated with the enhancement of resistivity, suggesting that a gap opens in the electron bands at $q=2k_{\text{F}}$. We also compare with a phenomenological model of the phase diagram. The richness of phases revealed in this study hint at the frustrated nature of the incommensurate magnetism present in EuAg$_4$Sb$_2$ and motivate further probes of these phases and the origin of the stability of spin moiré superlattices. Finally, the coupling of the multi-$q$ nature and $q=2k_{\text{F}}$ commensuration condition reveals the key requirements for a strong SMS transport response.

cond-mat.mes-hall↗

arXiv2025

Single-$q$ Cycloid and Double-$q$ Vortex Lattices in Layered Magnetic Semimetal EuAg$_4$Sb$_2$

Recently, a host of exotic magnetic textures such as topologically protected skyrmion lattices has been discovered in several bulk metallic lanthanide compounds. In addition to hosting skyrmion phases, a hallmark of this class of materials is the appearance of numerous spin textures characterized by a superposition of multi-$q$ magnetic modulations: spin moiré superlattices. The nuanced energy landscape thus motivates detailed studies to understand the underlying interactions. Here, we comprehensively characterize and model the three zero-field magnetic textures present in one such material, EuAg$_4$Sb$_2$. Systematic symmetry breaking experiments using magnetic field and strain determine that the ground state incommensurate magnetic phase (ICM1) is single-$q$. In contrast, ICM2 and ICM3 are both double-$q$, \textit{i.e.}, spin moiré superlattices. Further, through application of polarized small angle neutron scattering and spherical neutron polarimetry, we demonstrate that ICM1 is a single-$q$ cycloid and ICM2 and ICM3 are double-$q$ vortex lattices, with Eu moments lying in the $ab$-plane in zero field and with a ferromagnetic component at finite field. Despite the quasi-2D nature of EuAg$_4$Sb$_2$, the modulations propagate out of the \textit{ab}-plane, leading to a shift of the spin texture between triangle lattice planes. Further, the ICM3 to ICM2 transition includes an unusual 45$^\circ$ rotation of the magnetic vortex lattice. Motivated by the coexistence of such drastically different phases in this compound, we conclude by developing a phenomenological model to understand the stability of these states. Our experimental probes and theoretical modeling definitively characterize three different and tunable phases in one material, and provide insight for the design of new topological spin-texture materials.

cond-mat.mes-hall↗

arXiv2025

Symmetry classification of magnetic octupole current based on multipole representation theory

Magnetic octupole (MO) currents have recently attracted significant attention as a driving force for the Neel vector dynamics in d-wave altermagnets, a new class of antiferromagnets that exhibit nonrelativistic spin-split band structures. From a symmetry perspective, the MO includes an axial-dipole component analogous to that of the spin, making it essential to clarify how MO currents differ from spin currents. We here investigate the correspondence between MO conductivities and electronic multipoles, which provide a unified and powerful framework for symmetry analysis. We derive the multipole representation of the rank-five MO conductivity tensor and classify its symmetry-allowed components for all crystallographic point groups, in direct comparison with spin conductivity. We show that time-reversal-even electric-type multipoles give rise to the dissipationless MO current, whereas time-reversal-odd magnetic-type multipoles generate dissipative MO current under an applied electric field. Complementing this macroscopic analysis, the linear-response calculations for a microscopic tight-binding model demonstrate how MO conductivities are activated by symmetry lowering, exemplified by the symmetry reduction from Oh to Th. Our results elucidate the symmetry distinctions between MO currents and spin currents, and provide insights into their experimental identification.

cond-mat.str-el↗

arXiv2025

Large Anomalous Hall Effect in Topologically Trivial Double-$Q$ Magnets

Multi-$Q$ magnets consist of superposed spin density waves with distinct magnetic modulation vectors, enabling a wide range of magnetic orders depending on their combination. Among them, topologically nontrivial spin textures, such as a magnetic skyrmion, has been extensively studied owing to the emergence of topological Hall effects induced by real-space scalar spin chirality. Contrary to this expectation, we theoretically investigate another route to enhancing the Hall response under a topologically \textit{trivial} double-$Q$ spin textures. Despite the cancellation of the scalar spin chirality, the double-$Q$ magnetism exhibits a pronounced Hall response with a nonmonotonic dependence on the uniform magnetization, which is in stark contrast to a ferromagnetic state and a single-$Q$ spiral state. Analyzing the multi-orbital Kondo lattice model, we show that orbital hybridization induced by the double-$Q$ superstructure enhances the Berry curvature in $\mathbf{k}$-space, leading to a large anomalous Hall effect. This mechanism accounts for the observed giant anomalous Hall effect in GdRu$_2$Si$_2$ and GdRu$_2$Ge$_2$, thereby highlighting topologically trivial double-$Q$ spin textures as promising spintronic materials.

cond-mat.str-el↗

arXiv2025

Finite-temperature stability of skyrmion crystals in frustrated magnets: Role of sixfold anisotropy and uniform spin mode in momentum space

We study the finite-temperature stability of skyrmion crystals in frustrated magnets by analyzing the momentum-space exchange interaction of a classical Heisenberg model on a triangular lattice. Our analysis identifies two key momentum-space features that play a crucial role in stabilizing the skyrmion crystal phase. The first is the sixfold anisotropy in the momentum-space exchange interaction, which acts as a locking potential favoring triple-$Q$ skyrmion crystals. Monte Carlo simulations reveal that a larger anisotropy tends to enhance the stability region of the skyrmion crystal in the temperature--magnetic-field phase diagram. The second factor is the momentum-space energy related to the uniform spin mode, which correlates with the emergence of the skyrmion crystal phase at finite temperatures. These results provide a further understanding of the stabilization mechanism of the skyrmion crystal phase in frustrated magnets and will be useful for the design of skyrmion-hosting materials.

cond-mat.str-el↗