arXiv subjects

Satoru Hayami

Publications and source records attributed to Satoru Hayami.

At least 19 recordsLinked to original sources

arXiv2026

Spin-Point-Group Classification of Multipoles for Nonrelativistic Collinear Magnets

Multipole moments provide a unified symmetry language for describing electronic degrees of freedom and their associated physical responses. Although active multipoles have been systematically classified for crystallographic and magnetic point groups, their classification in spin point groups, which naturally describe magnetic systems in the absence of spin-orbit coupling, has remained unexplored. In this work, we present a complete classification of active orbital and spin multipoles for all 32 nonmagnetic and 122 collinear spin point groups. Treating orbital and spin degrees of freedom independently, we identify the symmetry-allowed multipoles associated with nonmagnetic and collinear magnetic orderings and clarify their hierarchy through comparisons among the corresponding spin point groups. The resulting classification provides a symmetry-based database that distinguishes structural and magnetic contributions to active multipoles and directly identifies microscopic order parameters, including those responsible for $d$-, $g$-, and $i$-wave altermagnetism. Furthermore, by classifying response tensors within the same framework, we establish a correspondence between active multipoles and symmetry-allowed physical responses. This correspondence systematically distinguishes nonrelativistic responses that survive without spin-orbit coupling from those requiring relativistic effects, providing a unified framework for understanding and predicting spin-dependent electromagnetic and transport phenomena in magnetic materials.

cond-mat.mes-hall↗

arXiv2026

Observation of g-wave altermagnetic multipole

Over the past few years, altermagnets have emerged as a new class of collinear magnets with broken time-reversal symmetry, offering novel opportunities for spintronics beyond conventional magnets. Rather than from net magnetization, as in ferromagnets, the unconventional time-reversal symmetry breaking of altermagnets originates from antiferroic magnetic dipoles locked to higher-order multipoles. Here we report the direct visualization of a $g$-wave altermagnetic multipole in the canonical altermagnet CrSb. Combining high-energy synchrotron X-ray diffraction with valence electron density (VED) analysis, we uncover a pronounced directional anisotropy of the VED distribution alternating between Cr sublattices. This evidences the antiferroic order of electric hexadecapoles predicted in $g$-wave altermagnets. Its coexistence with antiferroic magnetic dipoles induces ferroic magnetic multipoles, as probed by polarized neutron diffraction. We further identify a microscopic model of altermagnetism that directly relates the $g$-wave multipole and the $g$-wave spin splitting. Through direct observation and quantification of multipoles, this study provides a real-space fingerprint of altermagnetism and establishes a general probe of hidden multipole order in quantum materials.

cond-mat.str-el↗

arXiv2026

Thermodynamic Electric Toroidal Dipole and Intrinsic Longitudinal Spin Transport

Electric toroidal dipoles (ETDs) characterize ferroaxial order, yet their bulk definition in periodic crystals has remained elusive because conventional multipole operators involve the ill-defined position operator. Here we formulate a thermodynamic ETD by coupling a spatially varying electric field to the relativistic spin-induced electric polarization. The resulting expression is gauge invariant and provides a bulk order parameter for ferroaxial phases. We further establish a direct relation between the chemical-potential derivative of the ETD and the intrinsic longitudinal spin conductivity in insulating systems. To demonstrate the formulation, we construct a minimal ferroaxial extension of the Kane--Mele model. The ETD becomes finite exclusively in the ferroaxial phase and is strongly enhanced near a small band gap, accompanied by a sizable longitudinal spin current. Our results establish a thermodynamic theory of ETDs in crystalline solids and identify the longitudinal spin conductivity as a direct transport manifestation of ferroaxial order.

cond-mat.str-el↗

arXiv2026

Finite-size effects and interaction-driven crossovers in quarter-filled attractive Hubbard model: Exact diagonalization, DMRG and machine-learning analysis

We investigate the quarter-filled attractive Hubbard model on finite-width cylindrical lattices using exact diagonalization (ED), density-matrix renormalization group (DMRG) and unsupervised machine-learning-based techniques. Analysis of the ground-state energetics, local observables and correlation functions reveals a continuous interaction-driven crossover from weakly correlated fermions to a regime dominated by tightly bound singlet pairs. This crossover originates from the competition between kinetic-energy-driven fermionic itinerancy and interaction-driven onsite pair formation and exhibits behavior consistent with the BCS--BEC crossover in the thermodynamic limit. Hole-binding-energy calculations provide direct energetic evidence for pair formation: the two-hole binding energy remains negative throughout the attractive regime whereas three-hole binding emerges only at sufficiently strong attraction and exhibits pronounced finite-size dependence. To obtain an unbiased characterization of the correlation landscape, we apply principal component analysis (PCA) and uniform manifold approximation and projection (UMAP) to the real-space correlation matrices. PCA reveals a systematic redistribution of correlation variance whereas UMAP identifies a clear separation between weak- and strong-pairing regimes. Both machine-learning-based approaches independently identify the same crossover region inferred from conventional observables while providing an order-parameter-independent characterization of the underlying reorganization of many-body correlations. Finite-size scaling analyses of the pairing structure factor and the leading PCA variance ratio demonstrate that these signatures remain robust with increasing system size.

cond-mat.str-el↗

arXiv2026

Spatially Dispersive Second-Harmonic Generation in Ferroaxial Systems

Spatially dispersive second-harmonic generation (SHG) provides a powerful probe of centrosymmetric multipolar states beyond the electric-dipole approximation. We develop a gauge-consistent microscopic theory of spatially dispersive SHG that treats electric-quadrupole (EQ) and magnetic-dipole (MD) processes on equal footing. Applying the formulation to a minimal triangular cluster model with ferroaxial order, which is closely related to an electric toroidal dipole, we show that the nonlinear optical response directly reflects the ferroaxial order parameter. The agreement between length- and velocity-gauge calculations confirms the gauge consistency of the formulation. We further demonstrate that the EQ and MD contributions exhibit different spectral weights despite sharing the same resonance energies. In particular, the MD channel can dominate over the EQ channel at selected resonances, indicating that it is not merely a perturbative correction. Our results establish that both EQ and MD processes are essential for a quantitative description of spatially dispersive SHG in ferroaxial materials and provide a microscopic basis for interpreting nonlinear optical signatures of centrosymmetric multipolar order.

cond-mat.str-el↗

arXiv2026

Microscopic Conversion of Structural Chirality into Electronic Chirality in Artificial Chiral Clusters

We investigate how structural chirality is converted into electronic chirality in a two-dimensional cluster model consisting of a triangular-lattice core and three rod-like extensions under a surface-induced polar field. By evaluating the electric toroidal monopole (ETM) and the Edelstein effect, we show that both quantities exhibit antisymmetric and nonmonotonic dependence on the structural parameter controlling the cluster geometry. While the total density of states is almost unaffected by the structural deformation, the ETM-resolved spectrum changes significantly, indicating that chirality is encoded in the symmetry character of the electronic states rather than in the overall electronic spectrum. Real-space analysis reveals that the induced ETM is concentrated near the vertices and rods, whereas the Edelstein response extends more broadly into the core region. Furthermore, a parameter-decomposition analysis identifies spin-orbit coupling, surface-induced parity mixing, and structural chirality as the essential ingredients for both phenomena. Our results clarify the microscopic connection between structural chirality, electronic chirality, and chiral transport responses in artificial surface nanostructures.

cond-mat.mes-hall↗

arXiv2026

Magnetic quadrupole current generation and accumulation in noncentrosymmetric systems

Magnetization control via magnetic octupole injection has recently been proposed for a new class of centrosymmetric antiferromagnets, namely $d$-wave altermagnets, where the magnetic octupole is the lowest-rank magnetic multipole allowed by symmetry and serves as an alternative carrier to spin injection. In contrast, in noncentrosymmetric antiferromagnets, the magnetic quadrupole (MQ) constitutes the lowest-rank symmetry-allowed magnetic multipole, suggesting that MQ currents can provide an efficient route toward magnetization control through MQ injection. Here, we establish the symmetry conditions for MQ-current generation by constructing the multipole representation of the MQ conductivity tensor and show that MQ currents are generically allowed in noncentrosymmetric crystallographic point groups. As a representative example, we demonstrate MQ-current generation in the linear-response regime associated with symmetry lowering from the centrosymmetric point group ($mmm$) to its noncentrosymmetric subgroup ($mm2$). Furthermore, we reveal MQ accumulation near sample edges, analogous to spin accumulation induced by the spin Hall effect. This edge accumulation provides direct evidence of MQ-current generation and constitutes a key prerequisite for realizing MQ injection and MQ-based magnetization control in noncentrosymmetric antiferromagnets.

cond-mat.str-el↗

arXiv2026

Sixteenfold Classification of Many-Body Multipoles from Rotation-Compatible Canonical Symmetries

The conventional multipole framework provides a standard symmetry-based language for one-body electronic degrees of freedom, but fails to distinguish physically distinct sectors in many-body operator space. We show that canonical symmetries compatible with rotations provide this structure through two additional 2 labels associated with the particle-number gauge transformation G{\pi}/2 and the particle-hole transformation CA. These labels separate operators with different body numbers and particle-number changes, leading to a sixteenfold classification that organizes selection rules for nonzero expectation values, induced multipoles, and symmetry-allowed couplings in many-body multipole space.

cond-mat.str-el↗

arXiv2026

Piezoaxial coupling for strain-selected ferroaxial domain control

We formulate a symmetry-based hierarchy of strain-derived conjugate fields for ferroaxial order, and demonstrate strain-selected ferroaxial domain control using first-principles calculations. Since ferroaxial order is even under both spatial inversion and time reversal, ordinary electric and magnetic fields cannot serve as universal linear conjugate fields. Homogeneous strain, however, can generate symmetry-allowed piezoaxial fields whose leading order is determined by the parent point group and by the chosen ferroaxial-axis component. For basal-plane strain, the leading field is linear in orthorhombic systems, quadratic in tetragonal systems, and cubic in trigonal and hexagonal systems. Cubic parent groups further split into two classes: cubic-I groups, $23$ and $m\bar{3}$, allow linear full-strain fields for selected axes, whereas cubic-II groups, $432$, $\bar{4}3m$, and $m\bar{3}m$, forbid linear fields and require quadratic or cubic strain combinations depending on the selected axis. In trigonal systems, the basal-plane deviatoric strain with signed amplitude $\varepsilon_{\rm u}$ and principal-axis angle $\theta$ gives the single-axis field $h\propto\varepsilon_{\rm u}^3\sin6\theta$. First-principles calculations for the trigonal ferroaxial compound Na$_2$BaMg(PO$_4$)$_2$ verify both the predicted angular dependence and cubic strain scaling of the ferroaxial domain splitting, and fixed-strain atomic relaxations show strain-selected evolution from the para-axial structure. These results establish static homogeneous strain as a symmetry-allowed conjugate field for ferroaxial order and suggest a route to ferroaxial domain control through strain-field cooling.

cond-mat.mtrl-sci↗

arXiv2026

Topological Weyl Phase of an Ideal Spin-Gapless Semiconductor KCrSe

The coexistence of topological and spin-polarized electronic states within a single material platform provides an attractive route toward emergent quantum phenomena and spintronic functionalities. However, materials simultaneously exhibiting spin-gapless semiconducting (SGS) behavior and Weyl semimetallicity remain exceedingly rare. Here, using first-principles calculations, we identify the half-Heusler compound KCrSe as an ideal spin-gapless Weyl semimetal. Transport calculations reveal a weak temperature dependence of the longitudinal conductivity and relatively small Seebeck coefficients, providing further evidence of its SGS nature. KCrSe hosts a single pair of Weyl nodes-the minimum number permitted in a Weyl semimetal-located in close proximity to the Fermi level (E$_\text{F}$), resulting in exceptionally clean bulk and surface electronic spectra. The nontrivial Berry curvature associated with these Weyl nodes gives rise to sizable anomalous transport responses, including an anomalous Hall conductivity of $\sigma_{xy}^{A}\sim 90.76~\mathrm{S\,cm^{-1}}$ and an anomalous Nernst conductivity of $\alpha_{xy}^{A}\sim 0.15~\mathrm{A\,m^{-1}K^{-1}}$ at E$_\text{F}$, with substantially enhanced values at lower energies. The combination of an ideal Weyl topology, fully spin-polarized low-energy states, and finite anomalous transport establishes KCrSe as a promising platform for designing high-efficiency topological spintronic devices.

cond-mat.mtrl-sci↗

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

Revisiting magnetoelectric response in collinear antiferromagnetic zigzag chains: A downfolding approach beyond conventional low-energy models

Magnetoelectric (ME) effects in antiferromagnets provide a fertile platform for exploring symmetry-driven cross-correlated responses. However, their microscopic origin remains elusive and is often obscured in simplified low-energy descriptions. In this study, we revisit the microscopic mechanism of the ME effect in a collinear antiferromagnetic zigzag chain by employing a multi-orbital tight-binding model that explicitly includes both $s$- and $p$-orbital degrees of freedom. Using analytical and numerical calculations based on the Kubo formula, we demonstrate that the ME response is governed by orbital degrees of freedom activated through $s$--$p$ hybridization, while the spin contribution vanishes due to spin conservation. To elucidate the low-energy description, we derive an effective Hamiltonian projected onto the $s$-orbital subspace using the Schur complement. We show that a naive application of the Kubo formula within this effective model fails to capture the ME response. This issue is resolved by systematically incorporating vertex corrections in terms of orbital hybridization into the response functions. Furthermore, by introducing a quasiparticle renormalization scheme, we formulate a renormalized Kubo formula that preserves conservation laws and accurately reproduces the full multi-orbital results. Our analysis revisits the conventional low-energy perspective and reveals that the ME effect originates from virtual interorbital processes encoded in vertex corrections, rather than from the bare low-energy Hamiltonian. The effective framework developed here provides a unified microscopic understanding of orbital-driven ME responses and offers a systematic route to incorporate hybridization effects beyond simple low-energy models.

cond-mat.str-el↗

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

Unconventional alternating out-of-plane spin polarization in the coplanar kagome antiferromagnet

The emergence of spin-polarized currents in nonrelativistic platforms continues to attract significant interest in spintronics. Here we demonstrate that a noncollinear kagome antiferromagnet can generate an alternating out-of-plane spin polarization originating from the spin chirality of the magnetic unit cell, in the absence of relativistic spin--orbit coupling. Under spatial confinement, the system develops distinct real-space spin separation patterns whose structure is governed by the symmetry of the lattice termination. In particular, breaking the transverse mirror symmetry of the ribbon produces an altermagnetic-like spin splitting in the band structure. Furthermore, we uncover a spin--edge locking mechanism in which propagating edge states acquire an unconventional spin polarization. These results highlight how magnetic symmetry and confinement can generate spin-polarized transport in coplanar antiferromagnets without relying on relativistic interactions.

cond-mat.mes-hall↗

arXiv2026

Magnetic toroidal monopoles from relativistic polarization responses to magnetic field gradients

The magnetic toroidal monopole, a time-reversal-odd scalar, has attracted attention through its characteristic responses, such as electric-field-induced nonreciprocal directional dichroism observed in Co$_2$SiO$_4$. However, its evaluation in crystalline solids remains unresolved, as it cannot be defined within conventional multipole expansions or thermodynamic formulations. In this paper, we propose a theoretical framework to evaluate the magnetic toroidal monopole in periodic crystals based on the response of relativistic electric polarization to a magnetic field gradient. By incorporating the magnetic-field-gradient correction to the relativistic polarization, we derive an explicit expression for the magnetic toroidal monopole beyond symmetry arguments. The resulting expression is formulated in terms of geometric quantity such as Berry curvatures and orbital magnetic moment defined in an extended parameter space spanning momentum, magnetic field, and electric field. We further perform model calculations for an antiferromagnetic system hosting a magnetic toroidal monopole and confirm that the proposed quantity is finite. These results provide a practical route to characterize magnetic toroidal monopoles in crystalline solids and clarify their quantum geometric nature.

cond-mat.str-el↗

arXiv2026

Thermodynamic Multipoles and Dissipative Conductivities in Metallic Systems

Multipoles provide a systematic framework for describing the electronic structures of quantum materials from a symmetry perspective. Thermodynamic multipole moments in crystalline solids exhibit direct microscopic connections to certain allowed physical responses beyond symmetry; however, such relations have thus far been limited to dissipationless responses in equilibrium insulating systems. Here, this framework is extended at a heuristic level by focusing on the Fermi-surface contributions to thermodynamic multipole moments. These contributions establish direct relations to dissipative transport responses characteristic of metals, including charge and spin conductivities. A key consequence is that the conductivities exhibit extrema, typically maxima, at chemical potentials where the corresponding Fermi-surface contributions to the multipoles vanish, specifically, the electric quadrupole for charge conductivity and the magnetic octupole for spin conductivity. These findings uncover a previously overlooked aspect of thermodynamic multipole moments, opening a new perspective on dissipative transport in metallic systems.

cond-mat.mes-hall↗

arXiv2026

Theory of Many-Body Multipole Operators in Single-Centered Electron Systems: Two-Body Toroidal Monopoles in Spinless Orbitals

One-body multipole operators are defined as irreducible representations of rotational symmetry together with spatial-inversion and time-reversal symmetries, providing a systematic framework for classifying electronic internal degrees of freedom and for describing a wide variety of composite order parameters. While this formalism has been successfully established for the one-body operator space, a systematic classification of the many-body operator space, especially in interacting systems, remains an open challenge. In this paper, we extend the multipole formalism in the one-body operator space to the many-body operator space. By formulating fermionic creation and annihilation operators as spherical tensors and employing Clebsch-Gordan coupling combined with the exterior (Grassmann) algebra, we construct an irreducible decomposition of many-body operators that fully incorporates fermionic antisymmetrization. As a concrete application, we classify monopoles appearing in spinless many-body operators. In particular, we show that the electric toroidal monopole, a pseudoscalar breaking spatial-inversion symmetry, and the magnetic toroidal monopole, a time reversal-odd scalar, become active in spinless interacting many-body systems, although they are absent in the spinless one-body hybrid orbital space.

cond-mat.str-el↗

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.

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