arXiv subjects

Hikaru Watanabe

Publications and source records attributed to Hikaru Watanabe.

At least 19 recordsLinked to original sources

arXiv2026

Intrinsic longitudinal spin-conductivity from ferroaxial spin-symmetry breaking

The interplay of antiferromagnetic order, momentum-dependent Bloch spin-splitting, time-reversal (T), and parity (P) symmetries in non-relativistic systems has emerged as a central theme for spintronics. Two well-known examples are P-preserving and T-violating altermagnets and P-violating and T-preserving odd-parity magnets. Here we examine a class of coplanar AFMs with a translation invariant vector spin chiral (VSC) order that preserves both P and T symmetries but breaks the non-relativistic spin-rotational invariance. Naively, such a VSC order is not expected to exhibit unusual phenomena. Here we show that the spin-rotational symmetry breaking generated by this VSC order can take on multiple forms, including ferroaxial symmetry breaking. We further show that these states allow for non-relativistic and non-dissipative purely longitudinal (or purely transverse) spin-conductivities. These states also allow the generation of non-relativistic altermagnetic spin-splittings through circularly polarized light. We identify 16 candidate materials in the Magndata database for which our theory is applicable and provide effective microscopic models and DFT-based results that highlight the large emergent spin conductivities.

cond-mat.str-el↗

arXiv2026

Microscopic calculation of coherence lengths and magnetic penetration depth in multiband superconductors

We present an extended Ginzburg-Landau (GL) method for calculating the superconducting coherence length and magnetic penetration depth at temperatures well below the transition temperature $T_{\mathrm c}$. In contrast to conventional GL theory, which expands the free energy in both the order parameters and their gradients, our method applies a perturbative expansion only to the covariant-gradient terms, while retaining the full dependence on the superconducting order parameters. The coefficients of these terms are determined from finite differences of microscopic free energies evaluated at small imposed pair momenta. The method applies to both single-band and multiband superconductors and therefore provides a framework for incorporating more realistic electronic structures. For the models examined here, the extended GL method agrees well with real-space Bogoliubov-de Gennes (BdG) calculations over a wide temperature range, while requiring substantially less computational effort.

cond-mat.supr-con↗

arXiv2026

Circular Raman responses from angular-momentum inequivalence in CoSi

Circularly polarized Raman scattering in solids exhibits distinct phenomena such as Raman optical activity (ROA) and chiral-phonon-induced frequency splitting, whose relationship has remained unclear. Here we show that these seemingly different responses can be understood within a common framework based on the inequivalence of phonon states carrying opposite crystal angular momenta. Using helicity-resolved Raman spectroscopy of the chiral crystal CoSi, we find that ROA and frequency splitting arise from different symmetry channels, namely axial multipolar symmetry and structural chirality, respectively. First-principles calculations reproduce both effects and clarify their symmetry origins. These results establish angular-momentum inequivalence as a unifying principle of circular Raman responses and link helicity-resolved Raman spectroscopy to the angular-momentum structure of chiral phonons in topological materials.

physics.optics↗

arXiv2026

Raman Optical Activity Induced by Ferroaxial Order in NiTiO$_3$

Raman optical activity (ROA), the dependence of Raman intensity on the circular polarization of incident and scattered light, has traditionally been observed in chiral molecules and magnetic materials, where inversion or time-reversal symmetry is broken. Here we demonstrate that ROA can also arise in a centrosymmetric and non-magnetic ferroaxial crystal. Using circularly polarized Raman spectroscopy on single-crystalline NiTiO$_3$, we observed a pronounced ROA signal in the cross-circular polarization configurations, which correlates with the ferroaxial domain structure. Our symmetry analysis, first-principles calculations of phonons, and tight-binding model calculations reveal that the natural ROA originates from the ferroaxial order and persists even within the electric dipole approximation. These results establish ROA as a powerful probe of ferroaxial order in centrosymmetric systems.

cond-mat.mtrl-sci↗

arXiv2026

Dual-circular Raman optical activity of axial multipolar order

Multipolar order, such as octupolar order, is a key concept in condensed matter physics, particularly in light of elusive hidden orders. However, its experimental identification remains challenging due to the absence of direct coupling to conventional external stimuli. In this study, we propose that dual-circular Raman scattering serves as a probe of multipolar anisotropies. By combining symmetry analysis with microscopic calculations, we identify that both time-reversal-even ($θ$-even) and time-reversal-odd ($θ$-odd) axial multipolar phases exhibit the sizable Raman optical activity as a direct consequence of multipolar symmetry breaking. The quantitative significance of the proposed response is demonstrated by the first-principles study of pyrite, a prototypical axial octupolar material. Furthermore, we reveal that a multipolar phonon, a three-dimensional and alternating displacement resembling the chiral phonon, plays a vital role in the proposed optical phenomena. Our findings open a pathway for identifying multipolar orders in various materials through dual-circular Raman spectroscopy as a sensitive and versatile probe.

physics.optics↗

arXiv2026

Electric toroidal octupolar symmetry in pyrite FeS$_2$ probed by Raman optical activity

We report Raman optical activity in pyrite FeS$_2$, which hosts an electric toroidal octupolar symmetry. A clear and reproducible sign reversal of the circular intensity difference is observed between neighboring $\{111\}$ faces under cross-circular polarization. The signal appears only for the doubly degenerate $E_g$ phonon mode and is absent for other modes, consistent with symmetry analysis. First-principles calculations reproduce these features, establishing Raman optical activity as a probe of higher-rank axial multipolar symmetry.

cond-mat.mtrl-sci↗

arXiv2026

Ferroaxial magnets: time-reversal-even mirror symmetry violation from spin order

We investigate ferroaxial magnets, a new class of spin-order-driven multiferroic magnets in which magnetic ordering induces mirror-symmetry breaking while preserving both time-reversal and spatial-inversion symmetries. These systems exhibit a ferromagnet-like axial anisotropy that allows optical control of the ferroaxial polarization, while their macroscopic time-reversal symmetry makes them attractive for antiferromagnetic spintronics. Using spin crystallographic group analysis, we identify the candidate materials and the nonrelativistic ferroaxial nature stemming from the strong exchange splitting of magnets. Furthermore, a symmetry-based identification shows magnetic materials that host ferroaxial order and metallic conductivity, realizing the ferroaxial metal state that undergoes a ferroaxial phase transition while remaining metallic. As a direct probe for the ferroaxial metal, we propose a third-order nonlinear Hall effect originating from the transverse coupling between the electric field and Berry curvature dipole mediated by the ferroaxial anisotropy. Our results establish ferroaxial magnets as a platform for nonrelativistic multiferroicity and spintronic applications.

cond-mat.mtrl-sci↗

arXiv2026

Systematic Magnetic Structure Generation Based on Oriented Spin Space Groups: Formulation, Applications, and High-Throughput First-Principles Calculations

We propose a framework for generating magnetic structures, inspired by the concept of oriented spin space groups (SSGs): magnetic structures are first generated as totally symmetric representations of an SSG and are then rotated such that they belong to the maximal magnetic space group of the SSG, which we term spin-symmetry-adapted (SSA) structures and oriented SSA structures, respectively. This is a natural framework to enforce fixed magnetic moment magnitudes on the symmetry-equivalent sites as well as to exploit the spin-orbit coupling (SOC)-induced hierarchy of energy scales. To examine the present scheme, we analyze the MAGNDATA database and find that 77% of the reported structures are reproducible at the SSG level, among which 82% are fully reproduced within the oriented SSG scheme, regardless of their spin-only group types or propagation vectors. To quantitatively assess computational and predictive performance, we perform spin density functional theory calculations for 283 materials, first carrying out self-consistent calculations for SSA structures without SOC, followed by fixed-charge calculations including SOC for the descendant oriented SSA structures. The experimental magnetic structures are reproduced as energetically most stable in 82% of cases at the SSG level without SOC and in 76% of cases at the oriented SSG level with SOC, showing that the fixed-charge scheme enables accurate evaluation of SOC-induced energy differences at low computational cost. The characteristic energy scale among oriented SSA structures is only $\sim$0.29 meV per magnetic atom, about 300 times smaller than that of distinct SSA structures. These results demonstrate that oriented SSG-based enumeration, combined with the two-step calculations for SSA and oriented SSA structures, provides an efficient and robust route for large-scale magnetic-structure prediction.

cond-mat.mtrl-sci↗

arXiv2025

Symmetry analysis of cross-circular and parallel-circular Raman optical activity

The Raman scattering regarding the circularly-polarized incident and scattered lights is closely related to the circular activity of a given system. We investigate the symmetry of its activity, called the cross-circular and parallel-circular Raman optical activity. The analysis is systematically performed with the magnetic point groups and indicates that the response allows for a useful diagnosis of the symmetry of materials like chirality and (magneto-)axiality. It is also shown that the Stokes and anti-Stokes processes are related to each other by the conserved antiunitary symmetry for the time-reversal operation and that combined with the mirror reflection.

cond-mat.mtrl-sci↗

arXiv2025

Quantum geometry in low-energy linear and nonlinear optical responses of magnetic Rashba semiconductor (Ge,Mn)Te

Quantum geometry appears as a key factor in understanding the optical properties of quantum materials, with the anticipation on diverging or quantized responses near the Dirac and Weyl points. Here we investigate linear and nonlinear optical responses -- optical conductivity and injection current -- in a magnetic Rashba semiconductor in the mid-infrared region, with varying the Fermi energy across the Dirac point. We reveal that the linear optical conductivity reflects quantum metric, which remains finite irrespective of the diminishing joint density-of-states at lower photon energy. It is also confirmed that the magnetic injection current enhances depending on the energy of the Fermi level relative to the Dirac point. These optical spectra are nicely reproduced by our theoretical calculations with geometrical effects taken into account.

cond-mat.mtrl-sci↗

arXiv2025

Dirac charge in antiferromagnetic topological semimetals

Topological node of electronic bands can carry emergent charge degree of freedom such as the Berry curvature monopole of the Weyl semimetals, which results in intriguing transport and optical phenomena. In this study, we discuss the existence of the hidden "Dirac charge" and its detection via the photocurrent response in antiferromagnetic (AFM) Dirac semimetals. In light of the Berry curvature defined in the spin and spin-charge-mixed parameter space, we identify Dirac charges as sources or sinks of the Berry curvature in the generalized parameter space. We demonstrate that this Dirac charge can be detected via the photocurrent driven by the spin-charge-coupled motive force. By using real-time simulation, we find that the Dirac charge plays a significant role in the photocurrent generation in AFM Dirac semimetals. This work reveals the hidden property of the Dirac points in AFM Dirac semimetals.

cond-mat.mes-hall↗

arXiv2025

Symmetry, microscopy and spectroscopy signatures of altermagnetism

Altermagnetism is a collinear compensated magnetically-ordered phase with a d, g or i-wave anisotropy and alternating spin polarization of the electronic structure in the position and momentum space. Its recent discovery was in part motivated by the research of compensated magnets towards highly scalable spintronic technologies. Simultaneously, altermagnetism shares the anisotropic higher-partial-wave nature of ordering with unconventional superfluid phases which have been at the forefront of research for the past several decades. These examples illustrate the interest in altermagnetism from a broad range of science and technology perspectives. After summarizing the diverse research context, we turn the focus of this review to the symmetry, microscopy and spectroscopy signatures of altermagnetism. We start from the description of spontaneously broken and retained symmetries which delineate the compensated altermagnetic ordering as a distinct magnetic phase. Next we focus on microscopic signatures and ordering mechanism of the altermagnetic phase. We highlight crystal-structure realizations of a characteristic ferroic order of anisotropic higher-partial-wave components of atomic-scale spin densities in altermagnets, ranging from weakly-interacting metals to strongly correlated insulators. The symmetry and microscopy signatures of altermagnetism are directly reflected in spin-dependent electronic spectra and responses. We review salient band-structure features originating from the altermagnetic ordering, and from its interplay with spin-orbit coupling and topological phenomena. Throughout the review we compare altermagnetism to traditional ferromagnetism and Neel antiferromagntism, and to the currently intensely explored magnetic phases with non-collinear symmetry-protected compensated spin orders. We accompany the theoretical discussions by references to relevant experiments.

cond-mat.mtrl-sci↗

arXiv2025

Nonlinear Hall effect driven by spin-charge-coupled motive force

Parity-time-reversal symmetric ($\mathcal{PT}$-symmetric) magnets have garnered much attention due to their spin-charge coupled dynamics enriched by the parity-symmetry breaking. By real-time simulations, we study how localized spin dynamics can affect the nonlinear Hall effect in $\mathcal{PT}$-symmetric magnets. To identify the leading-order term, we derive analytical expressions for the second-order optical response and classify the contributions by considering their transformation properties under $\mathcal{PT}$ symmetry. Notably, our results reveal that the sizable contribution is attributed to the mixed dipole effect, which is analogous to the Berry curvature dipole term.

cond-mat.str-el↗

arXiv2025

Multiferroic collinear antiferromagnet with hidden altermagnetic split

Altermagnets exhibit nonrelativistic spin splitting due to the breaking of time-reversal symmetry and have been garnering significant attention as promising materials for spintronic applications. In contrast, conventional antiferromagnets without spin splitting seem not to have any symmetry breaking and have drawn less attention. However, we show that conventional antiferromagnets with a nonzero propagation vector (Q vector) bring about nontrivial symmetry breakings. The incompatibility between the Q vector and nonsymmorphic symmetry leads to macroscopic symmetry breaking without lifting spin degeneracy. Moreover, the hidden altermagnetic spin splitting in the electronic structure gives rise to various emergent responses. To examine our prediction, we perform first-principles calculations for MnS2 and investigate its multiferroic properties, such as nonlinear transport and optical activity. Our findings reveal unique properties in conventional antiferromagnets, providing another perspective for designing spintronic materials.

cond-mat.mtrl-sci↗

arXiv2024

Magnetic parity violation and parity-time-reversal-symmetric magnets

Parity-time-reversal symmetry ($\mathcal{PT}$ symmetry), a symmetry for the combined operations of space inversion ($\mathcal{P}$) and time reversal ($\mathcal{T}$), is a fundamental concept of physics and characterizes the functionality of materials as well as $\mathcal{P}$ and $\mathcal{T}$ symmetries. In particular, the $\mathcal{PT}$-symmetric systems can be found in the centrosymmetric crystals undergoing the parity-violating magnetic order which we call the odd-parity magnetic multipole order. While this spontaneous order leaves $\mathcal{PT}$ symmetry intact, the simultaneous violation of $\mathcal{P}$ and $\mathcal{T}$ symmetries gives rise to various emergent responses that are qualitatively different from those allowed by the nonmagnetic $\mathcal{P}$-symmetry breaking or by the ferromagnetic order. In this review, we introduce candidates hosting the intriguing spontaneous order and overview the characteristic physical responses. Various off-diagonal and/or nonreciprocal responses are identified, which are closely related to the unusual electronic structures such as hidden spin-momentum locking and asymmetric band dispersion.

cond-mat.mtrl-sci↗

arXiv2024

A study of centaur (54598) Bienor from multiple stellar occultations and rotational light curves

Centaurs, distinguished by their volatile-rich compositions, play a pivotal role in understanding the formation and evolution of the early solar system, as they represent remnants of the primordial material that populated the outer regions. Stellar occultations offer a means to investigate their physical properties, including shape, rotational state, or the potential presence of satellites and rings. This work aims to conduct a detailed study of the centaur (54598) Bienor through stellar occultations and rotational light curves from photometric data collected during recent years. We successfully predicted three stellar occultations by Bienor, which were observed from Japan, Eastern Europe, and the USA. In addition, we organized observational campaigns from Spain to obtain rotational light curves. At the same time, we develop software to generate synthetic light curves from three-dimensional shape models, enabling us to validate the outcomes through computer simulations. We resolve Bienor's projected ellipse for December 26, 2022, determine a prograde sense of rotation, and confirm an asymmetric rotational light curve. We also retrieve the axes of its triaxial ellipsoid shape as a = (127 $\pm$ 5) km, b = (55 $\pm$ 4) km, and c = (45 $\pm$ 4) km. Moreover, we refine the rotation period to 9.1736 $\pm$ 0.0002 hours and determine a geometric albedo of (6.5 $\pm$ 0.5) %, higher than previously determined by other methods. Finally, by comparing our findings with previous results and simulated rotational light curves, we analyze whether an irregular or contact-binary shape, the presence of an additional element such as a satellite, or significant albedo variations on Bienor's surface, may be present.

astro-ph.EP↗

arXiv2024

Generalized Pitaevskii relation between rectifying and linear responses: its application to reciprocal magnetization induction

Nonlinear optics has regained attention in recent years, especially in the context of optospintronics and topological materials. Nonlinear responses involved in various degrees of freedom manifest their intricacy more pronounced than linear responses. However, for a certain class of nonlinear responses, a connection can be established with linear-response coefficients, enabling the exploration of diverse nonlinear-response functionality in terms of the linear-response counterpart. Our study quantum-mechanically elucidates the relation between such nonlinear and linear responses we call the Pitevskii relation and identifies the condition for the relation to hold. Following the obtained general formulation, we systematically identify the Pitaevskii relations such as the inverse magnetoelectric effect and inverse natural optical activity unique to systems manifesting the space-inversion-symmetry breaking. These results provide a systematic understanding of intricate nonlinear responses and may offer further implications to ultrafast spintronics.

cond-mat.mtrl-sci↗

arXiv2024

Effect of collective spin excitation on electronic transport in topological spin texture

We develop an efficient real-time simulation method for the spin-charge coupled system in the velocity gauge. This method enables us to compute the real-time simulation for the two-dimensional system with the complex spin texture. We focus on the effect of the collective excitation of the localized spins on the electronic transport properties of the non-trivial topological state in real space. To investigate this effect, we calculate the linear optical conductivity by calculating the real-time evolution of the Kondo lattice model on the triangular lattice, which hosts an all-in/all-out magnetic structure. In the linear conductivity spectra, we observe multiple peaks below the bandgap regime, attributed to the resonant contributions of collective modes similar to the skyrmionic system, alongside broadband modifications resulting from off-resonant spin dynamics. The result shows that the collective excitation, similar to the skyrmionic system, influences the optical response of the electron systems based on symmetry analysis. We elucidate the interference between the contributions from the different spin excitations to the optical conductivity in the multiple spin texture, pointing out the mode-dependent electrical activity. We show the complex interplay between the complex spin texture and the itinerant electrons in the two-dimensional spin-charge coupled system.

cond-mat.str-el↗