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Satoru Hayami

Publications and source records attributed to Satoru Hayami.

At least 73 records · Page 4Linked to original sources
arXiv2024

Nonlinear frequency-asymmetric optical response in chiral systems

We report our theoretical results on the emergence of a nonlinear frequency-asymmetric optical response characteristic of chiral crystal systems with neither spatial inversion symmetry nor mirror symmetry. Based on the group theoretical analysis, we show that the chirality-related second-order nonlinear optical response occurs for two different input frequencies when the low-energy model Hamiltonian includes a time-reversal-even pseudoscalar quantity, i.e., the electric toroidal monopole. We demonstrate its emergence by investigating a fundamental microscopic model with the chiral-type antisymmetric spin--orbit interaction on a simple cubic lattice. By analyzing the behavior of nonlinear optical conductivity based on the Kubo formula, we find that the response is largely enhanced when one of the frequencies is set to zero and the other is set to a resonant frequency. We also discuss the relaxation time dependence of the response.

cond-mat.str-el↗
arXiv2024

Exploring topological spin order by inverse Hamiltonian design: A new stabilization mechanism for square skyrmion crystals

We propose a framework to construct a real-space spin model based on the inverse Hamiltonian design. The method provides an efficient way of realizing unconventional topological spin textures by optimizing the interaction parameters. In order to demonstrate its usefulness, we show that the tuning of the long-range exchange interactions can give rise to a square skyrmion crystal even without factors that have been previously identified as prerequisites for its stabilization, such as Dzyaloshinskii-Moriya interaction, multi-spin interaction, and bond-dependent magnetic anisotropy. Moreover, we elucidate the essence for the emergence of the square skyrmion crystal by classifying the parameter sets we get by the method. Since the present framework by adopting machine learning techniques can be universally applied to any magnetic system irrespective of lattice structures, it serves as the efficient construction of an effective spin model and the understanding of the stabilization mechanisms for unconventional topological spin orders.

cond-mat.str-el↗
arXiv2024

SP-STM study of the multi-Q phases in GdRu2Si2

The two stable surfaces of GdRu2Si2 are studied using spin-polarized scanning tunneling microscopy (SP-STM). Depending on the applied magnetic field different magnetic phases have been found and the presented measurements are in agreement with the respective previously proposed multi-Q spin textures. In particular the multi-Q nature of the zero magnetic field state, for which previous experiments could not rule out the coexistence of single-Q states, can be confirmed by our spin-resolved measurements on the Si-terminated surface. The surfaces of GdRu2Si2 exhibit strong magnetism-induced modulations of the spin-averaged density of states. We find that while the magnetic contribution to the tunnel signal can be clearly identified for the Si-terminated surface this proves to be much more difficult for the Gd-terminated surface. However, the magnetic field dependent spatial modulations on the Gd-terminated surface demonstrate that additional magnetic phase transitions occur for the surface layer compared to those identified for bulk GdRu2Si2 and possible spin textures are presented.

cond-mat.mtrl-sci↗
arXiv2024

Classification of multiorbital superconducting state based on augmented multipoles

The pairing interactions between electrons play an essential role in determining the properties in superconducting states. Recently, a plethora of unconventional superconducting states has been extensively explored, which often emerge owing to multipole fluctuations in the vicinity of multipole orders. We classify such superconducting states from the viewpoint of the multipole degrees of freedom by extending its representation to Nambu space. We clarify that under the crystallographic point group, arbitrary Cooper pairs between electrons with any angular momenta are systematically classified by four types of multipoles: electric, magnetic, magnetic toroidal, and electric toroidal. As examples, we apply our formulation to an $sp$-orbital electron system, which potentially exhibits exotic Cooper pairs under polar and axial point groups. Our systematic classification will be useful in characterizing unconventional superconducting states in multiorbital systems.

cond-mat.supr-con↗
arXiv2024

Unified description of electronic orderings and cross correlations by complete multipole representation

We overview recent developments of electronic orderings and associated cross correlations in condensed matter physics based on a complete set of multipole representations (electric, magnetic, electric toroidal, and magnetic toroidal multipoles) with distinct space-time inversion symmetries. By means of the symmetry-adapted complete basis set in any physical Hilbert space including atomic and cluster degrees of freedom, it provides a unified description of electronic orderings, deformations of band structures, cross correlations, and transport properties in solids. We review that a multipole representation is a powerful tool to attain a comprehensive understanding of various electronic properties and physical phenomena observed in materials. We also demonstrate how to identify active multipoles and expected physical phenomena in real materials with several examples. Our review would serve as a solid foundation for future studies toward observations and identifications of unknown electronic phases and their related physical phenomena.

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

Quantification of chirality based on electric toroidal monopole

Chirality ubiquitously appears in nature, however, its quantification remains obscure owing to the lack of microscopic description at the quantum-mechanical level. We propose a way of evaluating chirality in terms of electric toroidal monopole, a practical entity of time-reversal even pseudoscalar (parity-odd) object reflecting relevant electronic wave functions. For this purpose, we analyze a twisted methane at the quantum-mechanical level, showing that the electric toroidal monopoles become a quantitative indicator for chirality. In the twisted methane, we clarify that the handedness of chirality corresponds to the sign of the expectation value of the electric toroidal monopole, and that the most important ingredient is the modulation of the spin-dependent imaginary hopping between the hydrogen atoms, while the relativistic spin-orbit coupling within the carbon atom is irrelevant for chirality.

cond-mat.mtrl-sci↗
arXiv2024

Double-$Q$ and quadruple-$Q$ instabilities at low-symmetric ordering wave vectors under tetragonal symmetry

Multiple-$Q$ states are expressed as a superposition of spin density waves at multiple ordering wave vectors, which results in unconventional complicated spin textures, such as skyrmion, hedgehog, and vortex. We investigate the multiple-$Q$ instability by focusing on the low-symmetric ordering wave vectors in momentum space. By systematically performing the simulated annealing for effective spin models with various ordering wave vectors on a two-dimensional square lattice, we classify the magnetic phase diagram into four types according to the position of the ordering wave vectors. Three out of four cases lead to a plethora of isotropic multiple-$Q$ instabilities yielding collinear, coplanar, and noncoplanar double-$Q$ and quadruple-$Q$ magnetic phases, while the remaining case leads to an anisotropic double-$Q$ instability when the multiple-spin interaction is introduced. Our results indicate that exotic multiple-$Q$ phases distinct from the skyrmion crystal phase are expected when the ordering wave vectors lie on the low-symmetric positions in the Brillouin zone.

cond-mat.str-el↗
arXiv2024

Hybrid skyrmion and anti-skyrmion phases in polar $C_{\rm 4v}$ systems

We investigate the stability of the skyrmion crystal phase in a tetragonal polar system with the Dzyaloshinskii-Moriya interaction by focusing on the symmetry of ordering wave vectors forming the skyrmion crystal. Our analysis is based on numerical simulations for an effective spin model, which is derived from the weak-coupling regime in the Kondo lattice model on a polar square lattice. We show that a hybrid square skyrmion crystal consisting of Bloch and Néel spin textures emerges even under polar $C_{\rm 4v}$ symmetries when the ordering wave vectors correspond to low-symmetric wave vectors in momentum space, which is in contrast to the expectation from the Lifshitz invariants. We also show the instability toward the anti-skyrmion crystal and rhombic skyrmion crystal depending on the direction of the Dzyaloshinskii-Moriya vector in momentum space. Furthermore, we show that the regions of the skyrmion crystal phases are affected by taking into account the symmetric anisotropic exchange interaction. Our results open the potential direction of engineering the hybrid skyrmion crystal and anti-skyrmion crystal phases in polar magnets.

cond-mat.str-el↗
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

Field-induced transformation between triangular and square skyrmion crystals in a tetragonal polar magnet

Magnetic skyrmions with topologically nontrivial spin textures form a variety of periodic structures depending on microscopic interactions and lattice symmetry. We theoretically investigate a transformation between triangular and square skyrmion crystals against an external magnetic field in a polar tetragonal magnet. By performing the simulated annealing for a classical spin model, we show that the competition of the Dzyaloshinskii-Moriya interaction at multiple wave vectors is a key ingredient in inducing the structural transition in terms of the skyrmion crystals. The present results indicate the importance of magnetic frustration in momentum space as the origin of exotic topological phase transitions.

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

Magnetic instability under ferroaxial moment

Magnetic anisotropy is one of the important factors in determining magnetic structures. A type of magnetic anisotropy is closely related to the symmetry of crystals. We theoretically investigate magnetic anisotropy and its related magnetic instability arising from an electric axial moment, which appears under the breaking of the mirror symmetry parallel to the moment but does not require the breakings of both spatial inversion and time-reversal symmetries. By performing perturbation and mean-field calculations in a complementary way, we show the appearance of the in-plane magnetic anisotropy when the electric axial moment occurs, which tends to tilt in-plane spin moments from the crystal axes in collaboration with relativistic spin--orbit coupling. We demonstrate such a tendency for single-site and four-site cluster models, the latter of which leads to the instability toward a spin vortex phase accompanying magnetic monopole and magnetic toroidal dipole.

cond-mat.str-el↗
arXiv2023

Anisotropic skyrmion crystal on a centrosymmetric square lattice under an in-plane magnetic field

We investigate the instability toward a skyrmion crystal under an in-plane magnetic field in centrosymmetric tetragonal magnets. By performing the simulated annealing for a spin model on the square lattice, we find that the interplay among the easy-plane two-spin anisotropic interaction, positive biquadratic interaction, and high-harmonic wave-vector interaction plays an important role in stabilizing the skyrmion crystal in the in-plane external magnetic field. The obtained skyrmion crystal consists of a pair of vortex and antivortex with opposite polarization in terms of the out-of-plane spin component. We also show that various multiple-$Q$ states emerge depending on the model parameters. Our results indicate the possibility of topological spin textures in centrosymmetric tetragonal magnets with easy-plane magnetic anisotropy when the ground-state spin configuration at zero field is characterized by a single-$Q$ in-plane cycloidal spiral state.

cond-mat.str-el↗
arXiv2023

Classification of multipoles induced by external fields and currents under electronic nematic ordering with quadrupole moments

We theoretically investigate the effect of external fields and currents on electronic nematic orderings based on the concept of augmented multipoles consisting of electric, magnetic, magnetic toroidal, and electric toroidal multipoles. We show the relation between rank-2 electric quadrupoles and the other multipoles, the former of which corresponds to the microscopic order parameter for the nematic phases. The electric (magnetic) field induces the rank-1 and rank-3 electric (magnetic) multipoles and rank-2 electric toroidal (magnetic toroidal) quadrupoles, while the electric current induces the rank-1 and rank-3 magnetic toroidal multipoles and rank-2 magnetic quadrupoles. We classify the active multipoles under magnetic point groups, which will be a reference to explore cross-correlation and transport phenomena in nematic phases.

cond-mat.str-el↗
arXiv2023

Rotational Response Induced by Electric Toroidal Dipole

A ferroaxial ordering, which appears without mirror symmetry parallel to an electric axial moment, is described by a ferroic alignment of the electric toroidal (ET) dipole rather than the conventional electric and magnetic dipoles. Although its emergence requires neither spatial inversion nor time-reversal symmetry breakings, unconventional transverse responses between the conjugate physical quantities have been proposed, which are qualitatively different from those in multiferroic systems without both spatial inversion and time-reversal symmetries. We theoretically investigate a general relationship between ferroaxial ordering and its characteristic response tensor. We show that various rotational responses corresponding to an antisymmetric tensor component are related to the ferroaxial ordering based on symmetry analysis. Among them, we propose that second-order nonlinear magnetostriction, where the strain is induced by a second-order magnetic field, is one of the experimental setups to identify the ferroaxial ordering. We show its temperature and magnetic-field-angle dependence by analyzing a fundamental $d$-orbital model under the tetragonal symmetry.

cond-mat.str-el↗
arXiv2023

Three-sublattice antiferro-type and ferri-type skyrmion crystals in centrosymmetric magnets

We numerically investigate the stability of the skyrmion crystals in a centrosymmetric lattice structure by focusing on the role of magnetic frustration arising from the multi-sublattice degree of freedom. By analyzing an effective three-sublattice spin model with the antiferromagnetic exchange interaction between different sublattices and the momentum-resolved exchange interaction between the same sublattice based on the simulated annealing, we find that various skyrmion crystal phases with sublattice-dependent skyrmion numbers are stabilized with and without an external magnetic field depending on the model parameters: two ferro-type skyrmion crystal phases with the skyrmion numbers of one and two, two ferri-type skyrmion crystal phases, and one antiferro-type skyrmion crystal phase. Our results indicate that the multi-sublattice degree of freedom brings a further intriguing possibility of inducing exotic skyrmion crystal phases.

cond-mat.str-el↗
arXiv2023

Time-reversal switching responses in antiferromagnets

We propose emergent time-reversal switching responses in antiferromagnets, which is triggered by an accompanying magnetic toroidal monopole, i.e., time-reversal odd scalar distinct from electric and magnetic monopoles. We show that simple collinear antiferromagnets exhibit unconventional responses to external electric and/or magnetic fields once magnetic symmetry accommodates the magnetic toroidal monopole. We specifically demonstrate that the emergence of the magnetic toroidal monopole in antiferromagnets enables us to control rotational distortion by an external magnetic field, switch vortex-type antiferromagnetic structure by an external electric field, and convert right/left-handedness in chirality by a composite electromagnetic field. We also present the symmetry conditions to induce the magnetic toroidal monopole and exhibit candidate materials including noncollinear antiferromagnets in order to stimulate experimental observations.

cond-mat.str-el↗