arXiv subjects

Koki Shinada

Publications and source records attributed to Koki Shinada.

11 recordsLinked to original sources

arXiv2026

Phase-Space Quantum Geometry Beyond Adiabatic Electron Dynamics

The geometry of electronic quantum states plays an important role in the equilibrium and transport properties of solids. While the Berry curvature is known to influence electron motion, recent work has shown that the quantum metric also affects the motion of electron wave packets beyond the adiabatic approximation. To connect this nonadiabatic dynamics to many-electron observables, we derive an equivalent semiclassical formulation, valid up to second order in $\hbar$. The resulting phase-space measure and kinetic equation incorporate the quantum metric over the full phase space, including its mixed real-momentum components. We show that, in spatially inhomogeneous systems, the full phase-space quantum metric contributes to electric polarization in insulators and generates an intrinsic linear Hall response in metals. As a concrete example, we study Dirac electrons subject to a magnetic texture and a background potential that vary periodically in space. In this model, the mixed components of the phase-space metric produce a Hall contribution controlled by the relative phase between the two modulations. This phase-sensitive response can remain finite even when the conventional anomalous Hall conductivity vanishes. More broadly, our formulation enables the systematic study of equilibrium and transport responses in systems whose quantum geometry involves both position and momentum.

cond-mat.mes-hall↗

arXiv2026

Riemannian geometric classification and emergent phenomena of magnetic textures

We propose a new classification of magnetic textures from the viewpoint of differential geometry. Magnetic textures are conventionally classified into collinear, coplanar, and noncoplanar magnets. These classes are typically characterized by the vector spin chirality (VSC) and the scalar spin chirality (SSC), which indicate noncollinearity and noncoplanarity, respectively. However, this conventional classification is incomplete: in particular, noncoplanar textures cannot be fully characterized by the SSC alone, as exemplified by conical magnets. To refine this classification, we analyze the curves and surfaces traced by spins in real space using differential geometry and introduce two novel scalar spin chiralities that properly characterize noncoplanarity: the geodesic scalar spin chirality and the torsional scalar spin chirality. These quantities are directly connected to differential geometry: the former reflects the geodesic curvature while the latter is related to the torsion. Based on these chiralities, we identify three distinct classes of noncoplanar magnetic textures. Furthermore, analogous to the roles of the VSC and the conventional SSC in emergent electrodynamics, the geodesic SSC gives rise to novel emergent phenomena. By constructing a semiclassical theory including nonadiabatic effects and higher-order spatial gradients of magnetic textures, we demonstrate that the geodesic SSC induces an emergent band asymmetry, leading to nonreciprocal responses as a quantum geometric effect. This mechanism is a purely orbital effect, requiring no spin-orbit coupling, and the resulting discussion runs in parallel with the conventional picture of the topological Hall effect driven by the SSC. The geometric viewpoint developed here will provide broad new insights into classification, quantum geometry, emergent electrodynamics, and a wider variety of emergent phenomena.

cond-mat.mtrl-sci↗

arXiv2025

Quantum geometric bounds for observables: Linear responses, Drude weight, and orbital magnetization

The quantum geometric tensor (QGT) provides nontrivial bounds among physical quantities, as exemplified by the metric-curvature inequality. In this paper, we investigate various bounds for different observables through certain generalizations of the QGT. First, we demonstrate that bounds hold for all linear responses, which are produced by a QGT extended to many-body states, finite temperature, and general parameter space. As an application, we show the thermodynamic inequality originating from the convexity of free energy can be further tightened. Second, we establish a bound between the Drude weight and the orbital magnetization. The equality is exactly satisfied in the Landau level system, and systems with nearly flat bands tend to approach equality as well. We apply the resulting inequality to two orbital ferromagnets and support that the twisted bilayer graphene system is close to the Landau level system. Moreover, we show that an analogous inequality also holds for a higher-order multipole, magnetic quadrupole. Finally, we discuss the analogy between the QGT and the uncertainty principle, emphasizing that the existence of nontrivial bounds necessarily reflects quantum effects.

cond-mat.mtrl-sci↗

arXiv2025

Thermodynamic formulation of the spin magnetic octupole moment in bulk crystals

The discovery of unconventional antiferromagnets, such as altermagnets, has drawn significant attention to higher-rank magnetic multipoles, particularly magnetic octupoles. Despite the advances in research, attempts to understand their microscopic properties remain limited due to the unbounded nature of the position operator in bulk crystals. In this paper, we address this problem by using a well-known thermodynamic approach and derive a formula for the spin magnetic octupole moment (SMOM) that can be used in bulk crystals. The resulting formula is gauge invariant and satisfies St\v{r}eda formulas that relate the SMOM to the spin magnetoelectric dipole-quadrupole susceptibilities. Furthermore, we apply this formula to several models and examine the fundamental properties of the SMOM. For example, in $d$-wave altermagnets, the nonrelativistic component of the SMOM, which is independent of spin-orbit coupling, is larger than the relativistic component, which is induced by spin-orbit coupling. These nonrelativistic SMOMs have the same microscopic origin as the nonrelativistic spin splitting that characterizes $d$-wave altermagnetism. Moreover, they exhibit a N\'{e}el vector dependence consistent with Landau theory for $d$-wave altermagnetism [Phys. Rev. Lett. $\textbf{132}$, 176702 (2024)].

cond-mat.mtrl-sci↗

arXiv2024

Thermodynamic relations for the Cooper pair's momentum in helical superconductors

Two thermodynamic relations are proposed for the exact measurement of the center-of-mass (COM) momentum of the Cooper pairs in helical superconductors. The first relation concerns the constraint on the change in the COM momentum in response to the variation in the magnetic field, which is linked to the superconducting Edelstein effect. The second relation is related to a first-order phase transition, showing that the jump of the COM momentum can be measured from the slope of the transition line on the phase diagram, where the supercurrent is used as a control parameter. These relations solve the difficulty of detecting the momentum, enabling broader and more precise exploration of noncentrosymmetric superconductors.

cond-mat.supr-con↗

arXiv2024

Nonlinear Magnetoelectric Effect under Magnetic Octupole Order: Its Application to a $d$-Wave Altermagnet and a Pyrochlore Lattice with All-In/All-Out Magnetic Order

Extensive investigation has recently been conducted into a new class of antiferromagnetic order known as magnetic octupole order. However, the high rank of octupoles makes it difficult to detect and manipulate them by using conventional methods such as the anomalous Hall effect. In this paper, we propose the nonlinear magnetoelectric effect (NMEE), a second-order response to an electric field that induces a spontaneous magnetization, as a finite response under magnetic octupole order. First, we classify the magnetic point groups to identify antiferromagnets with such order, and derive the NMEE tensor using quantum kinetic theory. Then, we confirm the effectiveness of the NMEE through model calculations for two specific examples: a $d$-wave altermagnet and a pyrochlore lattice with all-in/all-out magnetic order. In particular, the intrinsic NMEE exhibits a large response in a magnetic Weyl semimetal phase of the pyrochlore lattice. This enhanced response is explained by the fact that the response tensor involves the quantum metric, which is enhanced near Weyl points. Furthermore, our results show that the NMEE has a sizeable value that can be detected by the magneto-optical Kerr effect.

cond-mat.mtrl-sci↗

arXiv2024

Orbital optical activity in noncentrosymmetric metals and superconductors

We present the optical activity induced by the orbital magnetic moment in metals and superconductors using Green's function formalization. We show that an apparent singularity of the optical activity vanishes in the normal state; however, it remains finite in the superconducting state and is related to the superconducting Edelstein effect, ensuring the missing area measurement. Finally, we calculate the optical activity in a model Hamiltonian mimicking doped transition metal dichalcogenides to investigate its characteristic spectrum, and we analyze the Kerr effect to discuss a possibility to observe the optical activity in experiments.

cond-mat.mtrl-sci↗

arXiv2023

Unique properties of the optical activity in noncentrosymmetric superconductors: sum rule, missing area, and relation with the superconducting Edelstein effect

We present general properties of the optical activity in noncentrosymmetric materials, including superconductors. We derive a sum rule of the optical activity in general electric states and show that the summation of the spectrum is zero, which is independent of the details of electric states. The optical activity has a $\delta$-function singularity that vanishes in normal phases. However, the singularity emerges in superconducting phases, corresponding to the Meissner effect in the optical conductivity. The spectrum decreases by the superconducting gap and has a missing area compared to the normal phase. This area is exactly equivalent to the coefficient of the $\delta$-function singularity due to the universal sum rule. Furthermore, the coefficient is exactly equivalent to the superconducting Edelstein effect, which has not yet been observed in experiments. Thus, this measurement of the missing area offers an alternative way to observe the superconducting Edelstein effect.

cond-mat.mtrl-sci↗

arXiv2023

Quantum theory of the Intrinsic Orbital Magnetoelectric Effect in itinerant electron systems at finite temperatures

Magnetization can be induced by an electric field in systems without inversion symmetry $\mathcal{P}$ and time-reversal symmetry $\mathcal{T}$. This phenomenon is called the magnetoelectric (ME) effect. The spin ME effect has been actively studied in multiferroics. The orbital ME effect also exists and has been mainly discussed in topological insulators at zero temperature. In this paper, we study the intrinsic orbital ME response in metals at finite temperature using the Kubo formula. The intrinsic response originates from the Fermi sea and does not depend on the dissipation. Especially in systems with $\mathcal{PT}$-symmetry, the extrinsic orbital ME effect becomes zero, and the intrinsic ME effect is dominant. We apply the response tensor obtained in this work to a $\mathcal{PT}$-symmetric model Hamiltonian with antiferromagnetic loop current order demonstrating that the intrinsic ME effect is enhanced around the Dirac points.

cond-mat.mtrl-sci↗

arXiv2023

Orbital Gravito-Magnetoelectric response and Orbital magnetic quadrupole moment correction

The magnetoelectric effect has been actively studied in multiferroics since the first observation in an antiferromagnetic, $\mathrm{Cr_2O_3}$. This effect appears in systems without spatial inversion symmetry and time-reversal symmetry and is sensitive to detecting magnetic quadrupole moments. It is often discussed as inducing spin magnetizations; however, the orbital magnetoelectric effect in metals has recently attracted much attention since its observation in $\mathrm{MoS_2}$ and twisted bilayer graphene. In this work, we propose the full quantum formalism for the temperature gradient induced-orbital magnetoelectric effect (orbital gravito-ME effect). The effect consists of two parts, i.e., an extrinsic part and an intrinsic part. We demonstrate that the intrinsic part needs a correction from the orbital magnetic quadrupole moment besides the usual Kubo formula to avoid an unphysical divergence at zero temperature and to satisfy the Mott relation. Furthermore, we show the classification table with the magnetic point group for the intrinsic and extrinsic effects. Finally, we analyze the intrinsic part in a $\mathcal{PT}$-symmetric model exhibiting an orbital magnetization order, i.e., a loop current order, and demonstrate the enhancement near Dirac points. We believe that these results will contribute to the detection and usage of orbital magnetic moments beyond spin moments.

cond-mat.mtrl-sci↗

arXiv2021

Nonlinear response induced by Ferromagnetism in a Noncentrosymmetric Kondo Lattice system

Recently, nonlinear responses have been actively studied in both experiments and theory. Particularly interesting are inversion-symmetry broken systems, where an even-order nonlinear electrical conductivity can be nonzero, resulting in nonreciprocity. Second-order nonlinear conductivities attract much attention because of their sensitivity to detect inversion-symmetry breaking in materials and their functionalities. However, while the nonlinear response has been actively studied in noninteracting systems for a long time, the nonlinear response in strongly correlated materials is still poorly understood. This paper analyzes the nonlinear conductivity in a correlated noncentrosymmetric system, namely a Kondo lattice system with Rashba type spin-orbit coupling. We mainly focus on the ferromagnetic phase, in which the second-order nonlinear conductivity becomes finite. Remarkably, we find that the second-order conductivity becomes only finite perpendicular to the ferromagnetic magnetization and has a strong spin dependence; due to a gap at the Fermi energy for one spin direction, the linear and nonlinear conductivity is only finite for the ungapped spin direction. Finally, we analyze sign changes in the nonlinear conductivity, which can be explained by a combination of correlation effects and the energetic shift due to the occurring ferromagnetism.

cond-mat.str-el↗