arXiv subjects

Akimitsu Kirikoshi

Publications and source records attributed to Akimitsu Kirikoshi.

12 recordsLinked to original sources

arXiv2026

Nonlocal Kondo-exchange-driven intrinsic anomalous Hall effect in localized-$4f$ antiferromagnetic metals

The anomalous Hall effect in antiferromagnetic metals has attracted considerable attention. Most known realizations involve itinerant $d$ electrons that simultaneously mediate charge transport and magnetic order. Here, we focus on $f$-electron materials, where localized magnetic moments and conduction electrons are hosted in different orbitals. We develop a theoretical framework to describe the impact of localized antiferromagnetic order on itinerant electrons. Applying this approach to the recently discovered $4f$ antiferromagnetic metal $\mathrm{Ce}_{2}\mathrm{Cu}\mathrm{Ge}_{6}$, we identify the origin of both the intrinsic anomalous Hall conductivity and the spin splitting of the energy bands as spin-dependent intersite hopping induced by nonlocal Kondo exchange coupling, rather than a Zeeman-type effective field acting locally on the conduction electrons.

cond-mat.str-el↗

arXiv2026

Light-induced nonlinear Edelstein effect under ferroaxial ordering

Ferroaxial ordering, a spontaneous rotational distortion of the atomic arrangement, brings about a cross-product-type spin-orbit coupling (SOC) manifested as an electric toroidal dipole. We propose the light-induced nonlinear Edelstein effect (NLEE) -- a second-order optical response in which a static magnetization is induced by a time-dependent electric field -- as a promising probe of ferroaxial ordering. First, we elucidate the relationship between the NLEE tensor and the electric toroidal dipole. By decomposing the polarization modes of light, we find that both the linearly polarized and circularly polarized light couple to the electric toroidal dipole via distinct mechanisms. We then demonstrate the NLEE using a minimal model that incorporates ferroaxial ordering. Our analysis reveals that effective coupling between orbital magnetization and SOC induces spin magnetization. In particular, the spin magnetization is tilted owing to the electric toroidal dipole; the tilt angle reflects the ratio between the ferroaxial-origin SOC and the relativistic SOC.

physics.optics↗

arXiv2026

Magneto-optical imaging of macroscopic altermagnetic domains in MnTe

Altermagnets are a new class of magnets accompanying global time-reversal symmetry breaking (TRSB) without net magnetization. The TRSB results in formation of novel altermagnetic domains. Features of altermagnetic domains, in particular their responses to external stimuli, are essentially important but yet unexplored. Here, we report visualization of bulk altermagnetic domains in MnTe based on scanning magneto-optical Kerr-effect microscopy using telecom infrared wavelength. We found two distinct TRSB domains with large Kerr rotations that do not scale with its tiny bulk magnetization. We also revealed controllability and stability of domains against magnetic or thermal perturbations. Our first observation of altermagnetic domains using a laboratory-scale simple optical technique showing their movable nature provide firm bases for future fundamental and application studies of altermagnets.

cond-mat.mtrl-sci↗

arXiv2026

Multipolar fluctuations from localized 4f electrons in CeRh2As2

The heavy-fermion superconductor CeRh2As2 exhibits a non-superconducting phase transition that precedes the emergence of superconductivity. The nature of the corresponding order parameter remains under debate, with competing proposals involving magnetic dipoles or electric quadrupoles. We derive the momentum-dependent multipolar susceptibilities and effective interactions among the localized 4f electrons, based on the framework of density functional theory combined with dynamical mean-field theory. Magnetic fluctuations within the crystalline-electric-field (CEF) ground-state doublet are dominated by q=(1/2,1/2,0), corresponding to a two-dimensional checkerboard configuration of the magnetic moment M_z along the c axis. Hybridization between the CEF ground-state and the first-excited doublet gives rise to leading magnetic octupole fluctuations of z(x^2-y^2) symmetry, followed by electric quadrupole fluctuations of x^2-y^2 and {yz, zx} symmetries. By taking into account the anisotropic magnetic-field dependence of the transition temperature T_0, we conclude that an antiferromagnetic order of M_z at q=(1/2,1/2,0) is consistent with the experiments, owing to the enhancement of T_0 caused by fluctuations of the field-induced quadrupole of {yz, zx} type under an in-plane magnetic field.

cond-mat.str-el↗

arXiv2025

From localized 4f electrons to anisotropic exchange interactions in ferromagnetic CeRh6Ge4

CeRh6Ge4 is a cerium-based ferromagnetic material exhibiting a quantum critical behavior under pressure. We derive effective exchange interactions, using the framework of density functional theory combined with dynamical mean-field theory. Our results reveal that the nearest-neighbor ferromagnetic interaction along the c axis is isotropic in spin space, leading to a formation of spin chains. On the other hand, the inter-chain coupling is highly anisotropic: The in-plane moment weakly interacts ferromagnetically in the a--b plane to stabilize the ferromagnetic state, whereas the z-component couples antiferromagnetically, contributing to its destabilization. The magnetic anisotropy of the interchain interactions as well as of the local 4f wavefunctions characterizes the magnetic properties underlying the ferromagnetic transition and the quantum critical behavior in CeRh6Ge4.

cond-mat.str-el↗

arXiv2025

Finite-$q$ antiferrotoroidal and ferritoroidal order in a distorted kagome structure

A highly geometrically frustrated lattice structure such as a distorted kagome (or quasikagome) structure enriches physical phenomena through coupling with the electronic structure, topology, and magnetism. Recently, it has been reported that an intermetallic HoAgGe exhibits two distinct magnetic structures with the finite magnetic vector $q=(1/3,1/3,0)$: One is the partially ordered state in the intermediate-temperature region, and the other is the kagome spin ice state in the lowest-temperature region. We theoretically elucidate that the former is characterized by antiferrotoroidal ordering, while the latter is characterized by ferritoroidal ordering based on the multipole representation theory, which provides an opposite interpretation to previous studies. We also show how antiferrotoroidal and ferritoroidal orderings are microscopically formed by quantifying the magnetic toroidal moment activated in a multiorbital system. As a result, we find that the degree of distortion for the kagome structure plays a significant role in determining the nature of antiferrotoroidal and ferritoroidal orderings, which brings about the crossover between the antiferro-type and the ferri-type distributions of the magnetic toroidal dipole. We confirm such a tendency by evaluating the linear magnetoelectric effect. Our analysis can be applied irrespective of lattice structures and magnetic vectors without annoying the cluster origin.

cond-mat.str-el↗

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↗

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

Microscopic mechanism for intrinsic nonlinear anomalous Hall conductivity in noncollinear antiferromagnetic metals

We theoretically investigate an intrinsic nonlinear anomalous Hall effect (INAHE) in space-time ($\mathcal{PT}$) symmetric antiferromagnetic metals. The INAHE is characterized by an asymmetric and non-dissipative part of the second-order electric conductivity tensor in the clean limit in contrast to the Drude-type symmetric conductivity tensor with dissipation. By introducing a multipole description, we show that the emergence of the INAHE is due to active odd-parity magnetic quadrupoles or magnetic toroidal dipoles under magnetic orderings. In order to clarify the microscopic origin of the INAHE, we specifically consider a fundamental tight-binding model of a three-dimensional tetragonal system. We demonstrate that the INAHE arises from the effective coupling between magnetic ordering and antisymmetric spin--orbit interaction. We also discuss essential electron hopping paths driving the INAHE.

cond-mat.str-el↗

arXiv2019

Variational Approach to Many-Body Problems Incorporating Many-Body Effects at Finite Temperature

We develop a variational approach at finite temperature that incorporates many-body correlation self-consistently. The grand potential is constructed in terms of Green's function expressed by the variational parameters. We apply this formalism to weakly interacting Bose-Einstein condensates to incorporate the dynamical 3/2-body processes, which are considered important in the dynamical properties. The processes lower the free energy below the mean-field Hartree--Fock--Bogoliubov's value in the same way as a previous zero-temperature formalism. From our numerical results, the pair creation or annihilation processes neglected in the Popov--Shohno approximation are enhanced, particularly in the long wavelength region, owing to the many-body effects. Because the 3/2-body correlations give a finite contribution to the self-energy of quasiparticles, they may change the microscopic properties qualitatively, even in the weak-coupling region.

cond-mat.quant-gas↗

arXiv2018

Variational Wave Function for Inhomogeneous Bose--Einstein Condensate with 3/2-Body Correlations

We construct a variational wave function for inhomogeneous weakly interacting Bose--Einstein condensates beyond the mean-field approximation by incorporating $3/2$-body correlations. From our numerical results calculated for a system trapped by a one-dimensional harmonic oscillator, the $3/2$-body correlations give a contribution comparable to the mean- field energy toward lowering the ground-state energy.

cond-mat.quant-gas↗

arXiv2018

Ground-State Wave Function with Interactions between Different Species in $M$-Component Miscible Bose-Einstein Condensates

We construct a variational ground-state wave function of weakly interacting M-component Bose-Einstein condensates beyond the mean-field theory by incorporating the dynamical 3/2-body processes, where one of the two colliding particles drops into the condensate and vice versa. Our numerical results with various masses and particle numbers show that the 3/2-body processes between different particles make finite contributions to lowering the ground-state energy, implying that many-body correlation effects between different particles are essential even in the weak-coupling regime of the Bose--Einstein condensates. We also consider the stability condition for $2$-component miscible states using the new ground-state wave function. Through this calculation, we obtain the relation $U^{2}_{AB}/U_{AA}U_{BB}<1+α$, where $U_{ij}$ is the effective contact potential between particles $i$ and $j$ and $α$ is the correction, which originates from the $3/2$-body and $2$-body processes.

cond-mat.quant-gas↗