arXiv subjects

Ryota Yambe

Publications and source records attributed to Ryota Yambe.

At least 19 recordsLinked to original sources

arXiv2025

Topological phase transitions by time-dependent electromagnetic fields in frustrated magnets: Role of dynamical and static magnetic fields

We theoretically investigate the effects of time-dependent electromagnetic fields on frustrated magnets with the spatial inversion symmetry. Two types of external-field setups are considered: One is a circularly polarized electromagnetic field and the other is a combination of a circularly polarized electric field and a static magnetic field. The system is modeled by a classical frustrated Heisenberg model on a triangular lattice, whose ground state is a single-$Q$ spiral spin configuration. The effects of irradiated electric and magnetic fields are taken into account by the inverse Dzyaloshinskii-Moriya (DM) interaction and the Zeeman coupling, respectively, without heating effects. By numerically solving the Landau-Lifshitz-Gilbert equation, we find that the two field configurations lead to distinct skyrmion crystal (SkX) phases and their associated topological phase transitions: in the former setup, SkXs composed of skyrmions with skyrmion numbers of one and two with opposite signs emerge, whereas in the latter setup, SkXs with the same sign appear. The stabilization mechanisms of these SkXs are accounted for by the competition among electromagnetic-field-induced chiral DM interactions, electric-field-induced three-spin interactions, and the Zeeman coupling based on the high-frequency expansion within the Floquet formalism. Furthermore, for the latter setup, we find that the stability region of the SkX phase varies significantly depending on the timing of the application of the circularly polarized electric field and the static magnetic field. Our findings would broaden the possible routes to generate and control SkXs by time-dependent electromagnetic fields, advancing both the theoretical comprehension and experimental control of topological spin crystals.

cond-mat.str-el↗

arXiv2025

Symmetry Rules on Multipole Interactions under Crystallographic Point Groups and Application to Multiple-$Q$ Multipole States

Multipole degrees of freedom describe the mutual interplay among the charge, spin, and orbital degrees of freedom in electrons, which provides a microscopic understanding of unconventional electronic orderings and their associated physical phenomena. We here show the symmetry rules on multipole interactions under crystallographic point groups in a systematic manner. Depending on the bond symmetries, we show the necessary symmetry conditions of the antisymmetric multipole interactions, which correspond to the extension of the Dzyaloshinskii-Moriya interaction, as well as the symmetric ones, which correspond to the extension of the compasslike interaction. Furthermore, we demonstrate that the symmetry-allowed multipole interactions can become a source of exotic multiple-$Q$ multipole orderings. As a specific example, we analyze the effective model with the antisymmetric quadrupole interaction on a triangular lattice and show the emergence of the triple-$Q$ quadrupole state. Our results indicate that multipole interactions that often arise from the heavy-fermion, frustrated, and nematic systems can potentially induce further unconventional quantum states of matter.

cond-mat.str-el↗

arXiv2025

Bimeron Crystals by a Linearly Polarized AC Electric Field in Frustrated Magnets

We theoretically propose a method to generate topological spin textures by irradiating a classical spin system with a linearly polarized AC electric field. To this end, we investigate non-equilibrium steady states in a classical Heisenberg model with frustrated exchange interactions on a two-dimensional triangular lattice by numerically solving the Landau-Lifshitz-Gilbert equation at zero temperature. Our results reveal that the linearly polarized AC electric-field irradiation induces a topological phase transition from a single-Q spiral state to a bimeron crystal with the skyrmion number of one in the low-frequency regime. Furthermore, we show that the obtained bimeron crystal remains relatively stable against both easy-axis and easy-plane single-ion anisotropies.

cond-mat.str-el↗

arXiv2024

Stabilization mechanisms of magnetic skyrmion crystal and multiple-$Q$ states based on momentum-resolved spin interactions

Multiple-$Q$ states as represented by a magnetic skyrmion crystal and hedgehog crystal have been extensively studied in recent years owing to their unconventional physical properties. The materials hosting multiple-$Q$ states have been so far observed in a variety of lattice structures and chemical compositions, which indicates rich stabilization mechanisms inducing the multiple-$Q$ states. We review recent developments in the research of the stabilization mechanisms of such multiple-$Q$ states with an emphasis on the microscopic spin interactions in momentum space. We show that an effective momentum-resolved spin model is a canonical model for not only understanding the microscopic origin of various multiple-$Q$ states but also exploring further exotic multiple-$Q$ states with topological properties. We introduce several key ingredients to realize the magnetic skyrmion crystal with the skyrmion numbers of one and two, hedgehog crystal, meron-antimeron crystal, bubble crystal, and other multiple-$Q$ states. We also review that the effective spin model can be used to reproduce the magnetic phase diagram in experiments efficiently.

cond-mat.str-el↗

arXiv2024

Dynamical generation of skyrmion and bimeron crystals by a circularly polarized electric field in frustrated magnets

A skyrmion crystal (SkX) has attracted much attention in condensed matter physics, since topologically nontrivial structures induce fascinating physical phenomena. The SkXs have been experimentally observed in a variety of materials, where the Zeeman coupling to the static magnetic field plays an important role in the formation of the SkXs. In this study, we theoretically propose another route to generate the SkXs by using a circularly polarized electric field. We investigate a non-equilibrium steady state in a classical frustrated Heisenberg magnet under the circularly polarized electric field, where the electric field is coupled to the electric polarization via the spin-current mechanism. By numerically solving the Landau-Lifshitz-Gilbert equation at zero temperature, we show that the electric field radiation generates a SkX with a high topological number in the high-frequency regime, where the sign of the skyrmion number is fixed to be negative (positive) under the left (right) circularly polarized field. The intense electric field melts these SkXs and generates isolated skyrmions. We clarify that the microscopic origin is effective electric-field-induced three-spin interactions by adopting the high-frequency expansion in the Floquet formalism. Furthermore, we find that the electric field radiation generates another type of SkXs, a bimeron crystal, in the low-frequency regime. Our results provide a way to generate the SkXs and control the topology by the circularly polarized electric field.

cond-mat.str-el↗

arXiv2024

Non-coplanar helimagnetism in the layered van-der-Waals metal DyTe$_3$

Magnetic materials with highly anisotropic chemical bonding can be exfoliated to realize ultrathin sheets or interfaces with highly controllable optical or spintronics responses, while also promising novel cross-correlation phenomena between electric polarization and the magnetic texture. The vast majority of these van-der-Waals magnets are collinear ferro-, ferri-, or antiferromagnets, with a particular scarcity of lattice-incommensurate helimagnets of defined left- or right-handed rotation sense, or helicity. Here we use polarized neutron scattering to reveal cycloidal, or conical, magnetic structures in DyTe$_3$, with coupled commensurate and incommensurate order parameters, where covalently bonded double-slabs of dysprosium square nets are separated by highly metallic tellurium layers. Based on this ground state and its evolution in a magnetic field as probed by small-angle neutron scattering (SANS), we establish a one-dimensional spin model with off-diagonal on-site terms, spatially modulated by the unconventional charge order in DyTe$_3$. The CDW-driven term couples to antiferromagnetism, or to the net magnetization in applied magnetic field, and creates a complex magnetic phase diagram indicative of competing interactions in an easily cleavable helimagnet. Our work paves the way for twistronics research, where helimagnetic layers can be combined to form complex spin textures on-demand, using the vast family of rare earth chalcogenides and beyond.

cond-mat.mtrl-sci↗

arXiv2023

Scalar spin chirality induced by a circularly polarized electric field in a classical kagome magnet

Noncoplanar magnetic states with a scalar spin chirality have been intensively studied in condensed matter physics, since they exhibit fascinating physical phenomena. We theoretically propose the generation of such noncoplanar magnetic states by using a circularly polarized electric field. By performing the micromagnetic simulation, we investigate a time evolution of a classical kagome magnet irradiated by the circularly polarized electric field. As a result, we find that the noncoplanar magnetic states are induced as a nonequilibrium steady state irrespective of the ground-state spin configurations. We show that the induced scalar spin chirality is controlled by the amplitude, frequency, and polarization of the electric field. In addition, we clarify that the mechanism of the noncoplanar magnetic states is accounted for by effective field-induced three-spin interactions by adopting the Floquet formalism in the high-frequency regime. We also show a condition to enhance the scalar spin chirality. Our results present a new reference for controlling the noncoplanar magnetic states and related phenomena by the circularly polarized electric field.

cond-mat.str-el↗

arXiv2023

Analysis of photo-induced chirality and magnetic toroidal moment based on Floquet formalism

We analyze the condition of photo-induced atomic-scale chirality and magnetic toroidal moment. By performing a high-frequency expansion in the Floquet formalism, we derive an effective static model Hamiltonian from the spinful $s$-$p$ hybridized model of a single atom interacting with an electromagnetic wave with a particular polarization. The lowest-order and third-order contributions in the high-frequency expansion give rise to the coupling to induce an electric toroidal monopole corresponding to microscopic chirality, while the second-order contribution provides the coupling to induce a magnetic toroidal dipole. We also discuss the condition of the polarization of the electromagnetic wave and induced multipoles. Our results stimulate a new direction of controlling unconventional multipoles by electromagnetic waves.

cond-mat.str-el↗

arXiv2023

Field-direction-dependent skyrmion crystals in noncentrosymmetric cubic magnets: A comparison between point groups $(O,T)$ and $T_{\rm d}$

We investigate the instability toward a skyrmion crystal (SkX) in noncentrosymmetric cubic magnets with an emphasis on a comparison between point groups $(O,T)$ and $T_{\rm d}$. By constructing low-temperature magnetic phase diagrams under an external magnetic field for three directions based on numerically simulated annealing, we find that the system under the point group $(O,T)$ exhibits different two types of SkXs depending on the field direction, while that under $T_{\rm d}$ does not show such an instability. The difference between them is understood from the difference in the momentum-dependent Dzyaloshinskii-Moriya interaction under each point group. Meanwhile, we show that the system under $T_{\rm d}$ leads to the SkX instability by considering an additional effect of the uniaxial strain, which lowers the symmetry to $D_{\rm 2d}$. We obtain two different SkXs: Néel-type and anti-type SkXs, the former of which is stabilized in the presence of the interactions at the three-dimensional ordering wave vectors. The present results provide rich topological spin textures in the three-dimensional systems, which are sensitive to the magnetic-field direction and point-group symmetry.

cond-mat.str-el↗

arXiv2023

Symmetry analysis of light-induced magnetic interactions via Floquet engineering

Anisotropic magnetic interactions become the origins of intriguing magnetic structures, such as helical and skyrmion structures by the Dzyaloshinskii-Moriya interaction. In general, possible anisotropic exchange interactions are restricted by crystal symmetry. Meanwhile, by lowering the crystal symmetry with light, additional anisotropic magnetic interactions are expected according to its polarization and frequency. In this study, we clarify a relationship between anisotropic magnetic interactions and symmetry lowering in insulating magnets irradiated by light. Based on the Floquet formalism, we find that a variety of anisotropic two-spin and three-spin interactions are induced via spin-dependent electric polarizations activated by light irrespective of the presence/absence of the spatial inversion symmetry; we systematically classify them in the hexagonal point group, tetragonal point group, and their subgroups. Our symmetry analyses show that the light-induced two-spin (three-spin) interaction is owing to the reduction of the point group to a chiral point group (black and white magnetic point group). We also demonstrate the effect of the light-induced magnetic interactions on the magnetic structures in a triangular unit. Our results will be a symmetry-based reference for the Floquet engineering of magnetic structures.

cond-mat.str-el↗

arXiv2023

Anisotropic spin model and multiple-$Q$ states in cubic systems

Multiple-$Q$ states manifest themselves in a variety of noncollinear and noncoplanar magnetic structures depending on the magnetic interactions and lattice structures. In particular, cubic-lattice systems can host a plethora of multiple-$Q$ states, such as magnetic skyrmion and hedgehog lattices. We here classify momentum-dependent anisotropic exchange interactions in the cubic-lattice systems based on the magnetic representation analysis. We construct an effective spin model for centrosymmetric cubic space groups, $Pm\bar{3}m$ and $Pm\bar{3}$, and noncentrosymmetric ones, $P\bar{4}3m$, $P432$, and $P23$: The former include the symmetric anisotropic exchange interaction, while the latter additionally include the Dzyaloshinskii-Moriya interaction. We demonstrate that the anisotropic exchange interaction becomes the origin of the multiple-$Q$ states by applying the anisotropic spin model to the case under $Pm\bar{3}$. We show several multiple-$Q$ instabilities in the ground state by performing simulated annealing. Our results will be a reference for not only exploring unknown multiple-$Q$ states but also understanding the origin of the multiple-$Q$ states observed in both noncentrosymmetric and centrosymmetric magnets like EuPtSi and SrFeO$_3$.

cond-mat.str-el↗

arXiv2022

Ferrochiral, antiferrochiral, and ferrichiral skyrmion crystals in an itinerant honeycomb magnet

Topological spin textures, such as a skyrmion crystal, are a source of unusual physical phenomena owing to the interplay between magnetism and topology. Since physical phenomena depend on the topological property and the symmetry of underlying spin structures, the search for new topological spin textures and emergent phenomena is one of the challenges in condensed matter physics. In this letter, we theoretically explore new topological spin textures arising from the synergy between spin, charge, and sublattice degrees of freedom in an itinerant magnet. By performing simulated annealing for an effective spin model of the honeycomb Kondo lattice model, we find a plethora of skyrmion crystal instabilities at low temperatures, whose topological spin textures are classified into three types: ferrochiral, antiferrochiral, and ferrichiral skyrmion crystals. We show that the obtained skyrmion crystals are the consequence of the spin-orbit-coupling-free honeycomb structure. Our results reveal the potential for itinerant honeycomb magnets to host a wide variety of SkXs and emergent phenomena.

cond-mat.str-el↗

arXiv2022

Engineering a skyrmion crystal in ferromagnetic/antiferromagnetic bilayers free from the DM interaction

We theoretically propose a new stabilization mechanism of a skyrmion crystal (SkX) in a bilayer triangular lattice system consisting of the ferromagnetic and the antiferromagnetic layers. By performing variational calculations and Monte Carlo simulations in a complementary way, we find that a magnetic frustration between the ferromagnetic and antiferromagnetic layers is a source of a finite-Q spiral state and the SkX in the strong interlayer coupling regime. We also show that the degree of frustration is related to the interlayer exchange interaction. The stronger interlayer coupling tends to make the effect of frustration larger, which results in the stabilization of the SkX. The present results not only provide a way of engineering the SkX in the ferromagnetic/antiferromagnetic domain and heterostructure but also imply the possibility of the SkX based on interorbital frustration scenario.

cond-mat.str-el↗

arXiv2022

Skyrmion crystal under $D_{3h}$ point group: Role of out-of-plane Dzyaloshinskii-Moriya interaction

The Dzyaloshinkii-Moriya (DM) interaction that originates from relativistic spin-orbit coupling in noncentrosymmetric magnets is a source of topological spin textures. We theoretically investigate the possibility of a skyrmion crystal by focusing on the role of the out-of-plane DM interaction that is different from the polar- and chiral-type DM interaction. By performing the simulated annealing for a spin model on a triangular lattice belonging to the $D_{3h}$ point group, we construct low-temperature magnetic phase diagrams in an applied magnetic field. As a result, we find the instability toward different skyrmion crystals under in-plane and out-of-plane magnetic fields, where the key ingredients to stabilize the SkX are different according to the magnetic field direction; the SkX is stabilized under the out-of-plane DM interaction for the in-plane magnetic field, while it is stabilized by additionally introducing an easy-axis anisotropy for the out-of-plane magnetic field. We also discuss the possible multiple-$Q$ states other than the skyrmion crystals in the presence of the out-of-plane DM interaction. Our result indicates that noncentrosymmetric magnetic materials with the out-of-plane DM interaction are potential candidates to host the skyrmion crystals.

cond-mat.str-el↗

arXiv2022

Helicity locking of square skyrmion crystal in a centrosymmetric lattice system without vertical mirror symmetry

We theoretically investigate the stability of a square skyrmion crystal (SkX) in a centrosymmetric tetragonal lattice structure with the emphasis on the role of the magnetic anisotropy arising from the absence of vertical mirror symmetry. Our analysis is based on an effective bilinear and biquadratic model in momentum space, which is a canonical model for itinerant magnets in a weak-coupling regime. By performing the simulated annealing for the model on the two-dimensional square lattice, we find that the off-diagonal spin component in the interaction, which becomes nonzero when the vertical mirror symmetry is broken, gives rise to the square SkX with a definite helicity in an external magnetic field. We show that the helicity of the centrosymmetric SkXs is determined by the competition between the off-diagonal and diagonal anisotropic interactions, the latter of which appears in the discrete fourfold-rotational lattice structure. Furthermore, we discuss helicity-dependent physical phenomena by introducing odd-parity multipoles, where electric (magnetic) and electric (magnetic) toroidal multipoles are sources of an antisymmetric spin polarization and an Edelstein effect (a magnetoelectric effect). We also discuss the stability of the SkXs with different helicities in a magnetic field rotation. Our results provide a way of engineering the helicity-locked SkXs by the symmetric anisotropic interaction in centrosymmetric magnets, which is distinct from that by the antisymmetric Dzyaloshinskii-Moriya interaction in noncentrosymmetric magnets.

cond-mat.str-el↗

arXiv2022

Effective spin model in momentum space: Toward a systematic understanding of multiple-$Q$ instability by momentum-resolved anisotropic exchange interactions

Multiple-$Q$ magnetic states, such as a skyrmion crystal, become a source of unusual transport phenomena and dynamics. Recent theoretical and experimental studies clarify that such multiple-$Q$ states ubiquitously appear under different crystal structures in metals and insulators. Toward a systematic understanding of the formation of the multiple-$Q$ states in various crystal systems, in this theoretical study, we present a low-energy effective spin model with anisotropic exchange interactions in momentum space. We summarize specific six symmetry rules for nonzero symmetric and antisymmetric anisotropic exchange interactions in momentum space, which are regarded as an extension of Moriya's rule. According to the rules, we construct the effective spin model for tetragonal, hexagonal, and trigonal magnets with crystal- and momentum-dependent anisotropic exchange interactions based on magnetic representation analysis. Furthermore, we describe the origin of the effective anisotropic exchange interactions in itinerant magnets by perturbatively analyzing a multi-band periodic Anderson model with the spin-orbit coupling. We apply the effective spin model to an itinerant magnet in a $P6/mmm$ crystal and find various multiple-$Q$ states with a spin scalar chirality in the ground state. Our results provide a foundation of constructing effective phenomenological spin models for any crystal systems hosting the multiple-$Q$ states, which will stimulate further exploration of exotic multiple-$Q$ states in materials with the spin-orbit coupling.

cond-mat.str-el↗

arXiv2021

Locking of skyrmion cores on a centrosymmetric discrete lattice: onsite versus offsite

A magnetic skyrmion crystal (SkX) with a swirling spin configuration, which is one of topological spin crystals as a consequence of an interference between multiple spin density waves, shows a variety of noncoplanar spin patterns depending on a way of superposing the waves. By focusing on a phase degree of freedom among the constituent waves in the SkX, we theoretically investigate a position of the skyrmion core on a discrete lattice, which is relevant with the symmetry of the SkX. The results are obtained for the double exchange (classical Kondo lattice) model on a discrete triangular lattice by the variational calculations. We find that the skyrmion cores in both two SkXs with the skyrmion number of one and two are locked at the interstitial site on the triangular lattice, while it is located at the onsite by introducing a relatively large easy-axis single-ion anisotropy. The variational parameters and the resultant Fermi surfaces in each SkX spin texture are also discussed. The different symmetry of the Fermi surfaces depending on the core position is obtained when the skyrmion crystal is commensurate with the lattice. The different Fermi-surface topology is directly distinguished by an electric probe of angle-resolved photoemission spectroscopy. Furthermore, we show that the SkXs obtained by the variational calculations are also confirmed by numerical simulations on the basis of the kernel polynomial method and the Langevin dynamics for the double exchange model and the simulated annealing for an effective spin model.

cond-mat.str-el↗

arXiv2021

Meron-antimeron crystals in noncentrosymmetric itinerant magnets on a triangular lattice

Multiple-$Q$ magnetic states often induce nontrivial topological spin textures, such as a skyrmion and a hedgehog. We theoretically investigate yet another multiple-$Q$ state with topological defects, a meron-antimeron crystal (MAX), represented by a periodic array of the meron and antimeron with a half-integer skyrmion number. Performing simulated annealing for an effective spin model of noncentrosymmetric itinerant magnets on a triangular lattice, we show that rectangular-shaped and triangular-shaped MAXs are stabilized by the interplay between the biquadratic interaction arising from the spin-charge coupling and the Dzyaloshinskii-Moriya interaction arising from the spin-orbit coupling. We also discuss the effect of a magnetic field on the triangular MAX, where highly anisotropic responses against a field direction are found. In particular, we show that the triangular MAX turns into the skyrmion crystal for the fields along the $y$ and $z$ directions, while it is replaced by another chiral state for the field along the $x$ direction. These results would inspire further experimental investigation of the MAXs in itinerant magnets.

cond-mat.str-el↗