arXiv subjects

Shintaro Hoshino

Publications and source records attributed to Shintaro Hoshino.

At least 19 recordsLinked to original sources

arXiv2026

Halogen control of magnetic competition in Kitaev candidate Ru$X_3$ ($X =$ Cl, Br)

The spin-orbital Mott insulators Ru$X_3$ ($X =$ Cl, Br) have attracted considerable attention as promising candidate materials for realizing a Kitaev spin liquid. In this study, we construct effective pseudospin models from multiorbital Hubbard models derived from first-principles calculations and investigate the magnetic states of RuCl$_3$ and RuBr$_3$. From the constructed effective models, we find that RuBr$_3$ has more extended Wannier orbitals and stronger interlayer exchange interactions than RuCl$_3$. These interactions enhance three-dimensional correlations, consistent with the stronger antiferromagnetic tendency experimentally inferred for RuBr$_3$. Orbital-dependent Coulomb anisotropy further reduces the energy difference between ferromagnetic and zigzag states. Our results clarify how halogen substitution controls magnetic competition in Ru$X_3$ through interlayer exchange interactions and effects of orbital-dependent Coulomb interactions.

cond-mat.str-el↗

arXiv2026

Electron-phonon-coupled Langevin dynamics for strongly-correlated insulators

The Landau-Lifshitz-Gilbert (LLG) equations are widely used to study spin dynamics in Mott insulators. However, because energy damping is typically introduced phenomenologically, their validity for describing nonequilibrium processes and their connection to the microscopic origin of dissipation in real materials remains unclear. In this paper, we derive generalized stochastic LLG equations from first principles for spin-orbital coupled Mott insulators, explicitly incorporating the coupling between electronic degrees of freedom and lattice vibrations. Our approach is based on a path-integral formalism formulated along the Keldysh contour, which naturally accounts for dissipation and thermal fluctuations through interactions with a phonon bath and emergent stochastic noise. We benchmark our theoretical framework by numerically integrating the equations of motion for a two-orbital spin chain coupled to Einstein phonons. The resulting energy relaxation mimics realistic cooling dynamics, exhibits nontrivial transient behavior during thermalization, and accurately reproduces thermodynamic properties upon equilibration. We further demonstrate how electron-phonon coupling induces hybridization between electronic and phononic modes in the excitation spectrum and show that the conventional LLG equations are recovered as a limiting case of our microscopic theory. These results establish a robust and reliable framework for capturing dissipative spin dynamics in strongly correlated systems, both in and out of equilibrium.

cond-mat.str-el↗

arXiv2026

Superconductivity-enhanced phonon angular momentum

We theoretically investigate the properties of phonon angular momentum in the superconducting state, using fulleride compounds in an external magnetic field as a model system. The electron orbital angular momentum injected by an external field is transferred to the phonon subsystem via electron--phonon coupling. We show that this field-induced phonon angular momentum is significantly enhanced and undergoes a sign reversal upon entering the superconducting state. In the normal state, the dominant energy scale governing the response function is the electronic bandwidth $D$. In the superconducting state, the phonon energy scale $ω_1$ enters the denominator, leading to an enhancement of order $D/ω_1$. The observed sign change in the response can be explained by the competition between Fermi surface and Fermi volume contributions.

cond-mat.supr-con↗

arXiv2026

Spherical-tensor description of the Jahn--Teller--Hubbard molecule and local electron--phonon entanglement

We investigate the localized-electron character of the Mott-insulating phase in A$_3$C$_{60}$ using a single-site multiorbital electron model coupled to anisotropic molecular vibrations (Jahn--Teller phonons). We apply the spherical-tensor formalism, a framework originally developed in nuclear physics, to analyze the electron--phonon-coupled ground-state multiplet. Focusing on multipole moments, we find that both the conventional electronic quadrupole moment and the lattice displacement associated with the molecular vibrations vanish, even though the degenerate ground-state multiplet implies the presence of quadrupolar degrees of freedom. By analyzing these degrees of freedom within the spherical-tensor framework, we introduce composite (two-body) quadrupole operators involving both electrons and phonons and study their parameter dependence numerically. Furthermore, using quasispin selection rules, we demonstrate that the composite quadrupole does not couple to either the conventional quadrupole or lattice-displacement operators, thereby distinguishing it fundamentally from standard quadrupolar degrees of freedom. In addition, we investigate the nature of the electron--phonon entanglement and characterize it from the viewpoint of angular momentum. Analysis of the entanglement spectrum reveals that the ground state consists of superpositions of multi-phonon states with angular momenta $L_{\rm ph}=2$ and $L_{\rm ph}=3$, formed through coupling to three-electron states with $L=1$ and $L=2$.

cond-mat.str-el↗

arXiv2026

Semiclassical representation of the Hubbard model

By revisiting the path-integral formulation of the Hubbard model, we propose a theoretical approach based on a semiclassical approximation employing an unconventional coherent-state representation. Within this framework, a subset of the dynamical variables is treated as static, yielding a nonperturbative scheme that is applicable at finite temperature, incorporates intersite correlations, and can be naturally extended to multiorbital systems. We assess the validity of the approximation by comparing its results with exact solutions for one- and two-site systems, focusing in particular on the particle number, double occupancy, hopping amplitude, and spin correlations, and find that the present approach qualitatively reproduces the exact behavior. Quantitatively, deviations arise, which is associated with the continuum (non-discretized) character of the underlying density of states. Furthermore, we derive the exact transformation associated with the coherent-state construction, thereby providing additional insight into the representation of the Hubbard model.

cond-mat.str-el↗

arXiv2026

Designing XY and Dzyaloshinskii--Moriya couplings in Majorana Cooper pair boxes

We theoretically study how to design spin couplings in networks of Majorana Cooper pair boxes (MCBs) connected by multiple normal-metal leads. The inter-box interaction is generated by the conduction-electron-mediated Ruderman--Kittel--Kasuya--Yosida (RKKY) interaction. We show that the connectivity of Majoranas to the leads enables arbitrary types of couplings. As concrete examples, we show the realization of the XY exchange interaction and the Dzyaloshinskii--Moriya (DM) interaction, which are difficult to implement in previously proposed MCB-based schemes. The sign and magnitude of the couplings can be tuned continuously via gate-controlled tunneling amplitudes. These results establish MCBs as a versatile platform for engineered quantum spin systems.

cond-mat.mes-hall↗

arXiv2026

Electron chirality and hydrodynamic helicity: Analysis in the atomic limit

Electron chirality has been proposed as a microscopic quantity that characterizes electronic handedness, yet its underlying control parameter has not been clearly identified. Furthermore, its applicability is limited to systems with spin-orbit coupling, which motivates the need for alternative measures of chirality. In this work, we explore two complementary measures of chirality: electron chirality and hydrodynamic helicity. By analyzing a minimal atomic model under chiral crystal fields, we clarify how the interplay among crystal fields, spin-orbit coupling, and electron correlation gives rise to non-zero values of chirality measures. Although electron chirality increases with both spin-orbit coupling and chiral crystal field strength, the dependence on these two factors is highly non-trivial. Particularly, when the chiral crystal field is varied continuously and the energy levels approach quasidegenerate points, the electron chirality is insensitive to spin-orbit coupling, resulting in a remarkable enhancement of chirality. In contrast, the hydrodynamic helicity, defined as a two-body pseudoscalar quantity, remains non-zero even without spin-orbit coupling, originating from electron-electron interactions. Perturbative analysis reveals distinct symmetry selection rules governing the two quantities. Our results provide fundamental insight into the origin of chiralities in electronic systems.

cond-mat.mtrl-sci↗

arXiv2025

Spin-stripes in the Hubbard model: a combined DMFT and Bethe-Salpeter analysis

The Hubbard model is known to accommodate various electronic orders, including stripes, which are important for understanding the physics of cuprates. We study spin-stripe order in the square lattice Hubbard model as a function of doping and temperature, by solving the Bethe-Salpeter equation with the local vertex from dynamical mean field theory (DMFT), both inside and outside the antiferromagnetic phase. We find broad regions of horizontal/vertical spin stripes at low temperatures for the model with and without next-nearest neighbor hopping. Their wavelength depends on hole doping in a nonlinear fashion, and is highly sensitive to the ratio of nearest and next-nearest neighbor hoppings.

cond-mat.str-el↗

arXiv2025

Nonlinear planar Hall effect from superconducting vortex motion

We report the nonreciprocal charge transport along the longitudinal and transverse directions in the vortex flow regime of FeSe superconducting films. Clear nonreciprocal signals under an inplane magnetic field reveals symmetry breaking at the film surfaces since the crystal structure of FeSe is centrosymmetric. Although the symmetry in such polar superconductors allows the nonreciprocal transverse response under a magnetic field parallel to the electric current, its observation is physically counterintuitive because vortex motion is not expected in this configuration. We propose that thermally excited (anti)vortices due to the two-dimensional nature of FeSe give rise to the nonreciprocal transverse signals when the mirror symmetry is broken by the inplane magnetic field.

cond-mat.supr-con↗

arXiv2025

Symmetry-breaking perturbations in the Jahn-Teller-Hubbard model

We study the effect of symmetry-breaking perturbations in the multiorbital Hubbard model coupled to anisotropic Jahn-Teller phonons, which is relevant for the description of fulleride superconductors. This system is often approximated by a model with static antiferromagnetic (AFM) Hund's coupling, in which the coupling to the Jahn-Teller phonon is effectively described, but the retardation effect associated with phonon propagation is neglected. We compare the properties of the models with static AFM Hund's coupling and dynamical Jahn-Teller electron-phonon interaction by means of the Eliashberg theory. Considering the susceptibilities for the spin, magnetic orbital, electric orbital, and superconductivity, we reveal a qualitatively different behavior between the two models in the case of the magnetic orbital susceptibility. We further study the effect of a magnetic field on the $s$-wave spin-singlet superconducting state. In the presence of the field, the magnetic orbital susceptibility becomes nonzero due to a combination of multiorbital and retardation effects, while the spin susceptibility remains zero at low temperatures. By analyzing this phenomenon both numerically and analytically, we clarify that odd-frequency pairs induced by the magnetic field play a crucial role in the spin and orbital magnetic susceptibilities. Thus, the magnetic degrees of freedom produce interesting behaviors in the presence of retardation effects associated with electron-phonon coupling.

cond-mat.supr-con↗

arXiv2025

Low-rank quantics tensor train representations of Feynman diagrams for multiorbital electron-phonon models

Feynman diagrams are an essential tool for simulating strongly correlated electron systems. However, stochastic quantum Monte Carlo sampling suffers from the sign problem, particularly when solving a multiorbital quantum impurity model. Recently, two approaches have been proposed for efficient numerical treatment of Feynman diagrams: Tensor Cross Interpolation (TCI) to replace stochastic sampling and the Quantics Tensor Train (QTT) representation for compressing space-time dependence. One of the remaining challenges is the nontrivial task of identifying low-rank structures in weak-coupling Feynman diagrams for multiorbital electron-phonon systems. In particular, the traditional TCI algorithm faces an ergodicity problem, which prevents it from fully exploring the multiorbital space. To address this, we incorporate a new algorithm called global search, which resolves this issue. By combining this approach with QTT, we uncover low-rank structures and achieve efficient numerical integration with exponential resolution in time and faster-than-power-law convergence of error relative to computational cost. Additionally, our approach does not require the division of discontinuous regions necessary in non-quantics TCI.

cond-mat.str-el↗

arXiv2025

Weak coupling approach to magnetic and orbital susceptibilities for superconducting states in multiorbital electron-phonon coupled model

Alkali-doped fullerides are molecular-based superconductors with multiple active orbitals. In this paper, using the Eliashberg theory with the retardation effect of Jahn-Teller phonons, we study the response of the spin-singlet superconducting state relevant to fulleride materials. The spin Zeeman field is not active for the singlet pairing state, and the magnetic orbital field, which physically generates a circular electron motion inside the fullerene molecule, is also shown to be inactive. On the other hand, the electric orbital (or quadrupolar) field, which corresponds to a uniaxial distortion, remains active across the superconducting phase transition. This is understood by the orbital-symmetric structure of the Cooper pair, which is susceptible to the electric orbital field, while it is not the case for the magnetic orbital field which tends to create an antisymmetric part.

cond-mat.supr-con↗

arXiv2024

Quantification of electronic asymmetry: chirality and axiality in solids

Chiral and axial materials offer platforms for intriguing phenomena, such as cross-correlated responses and chirality-induced spin selectivity. However, quantifying the properties of such materials has generally been considered challenging. Here, we demonstrate that the spatial distribution of the electron chirality, represented by $Ψ^\dagger γ^5 Ψ$ with the four-component Dirac field $Ψ$, characterizes the chirality and axiality of materials. Furthermore, we reveal that spin-derived electric polarization can serve as an effective indicator of material polarity. We present quantitative evaluations of electron chirality distribution and spin-derived electric polarization based on first-principles calculations. Additionally, we propose that electron chirality can be directly observed via circular dichroism in photoemission spectroscopy, which measures the difference between right- and left-handed circularly polarized light. Electron chirality and spin-derived electric polarization provide a new framework for quantifying chirality, axiality, and polarity in asymmetric materials, paving the way for the exploration of novel functional materials.

cond-mat.mtrl-sci↗

arXiv2024

Dirac bilinears in condensed matter physics: Relativistic correction for observables and conjugate electromagnetic fields

Inspired by recent developments in electron chirality, we reconsider some microscopic physical quantities that have been overlooked or have received little attention in condensed matter physics, based on the non-relativistic limit of the Dirac bilinears in relativistic quantum theory. We identify the expression of physical quantities defined by the four-component Dirac field in terms of the two-component Schrödinger field, which is usually used in condensed matter physics, and clarify its conjugate electromagnetic field. This consideration bridges the fields of condensed matter physics, quantum chemistry, and particle physics, and paves the way to electromagnetic control of matter. Our findings provide a means of {\it ab initio} quantification of material characters such as chirality and axiality that are unique to low-symmetry materials, and stimulate the systematic search for useful, new functionalities.

cond-mat.mes-hall↗

arXiv2024

Odd-frequency pairing of Bogoliubov quasiparticles in superconductor junction

We study a superconductor Josephson junction with a Bogoliubov Fermi surface, employing McMillan's Green's function technique. The low-energy degrees of freedom are described by spinless fermions (bogolons), where the characteristic feature appears as an odd-frequency pair potential. The differential equation of the Green's function is reduced to the eigenvalue problem of the non-Hermitian effective Hamiltonian. The physical quantities such as the density of states and pair amplitude are then extracted from the obtained Green's function. We find that the zero energy local density of states at the interface decreases as the relative phase of the Josephson junction increases. This decrease is accompanied by the generation of an even-frequency pair amplitude near the interface. We also clarify that the $π$-junction-like current phase relation is realized in terms of bogolons. In contrast to conventional $s$-wave superconductor junctions, where even-frequency pairs dominate in the bulk and odd-frequency pairs are generated near the interface, our findings illuminate the distinct behaviors of junctions with Bogoliubov Fermi surfaces. We further explore spatial dependences of these physical quantities systematically using quasiclassical Green's functions.

cond-mat.supr-con↗

arXiv2023

Generalized Slater-Condon parameters for relativistic strongly correlated orbitals

Relativistic correction to the Coulomb interaction is considered for strongly correlated electron orbitals. The atomic representation of the Coulomb-Breit interaction and its physical origin are clarified, to generalize a concept of the Slater-Condon parameters. In the derivation, it proves advantageous to employ the spherical representation of photons, which can be categorized into three types: scalar, electric, and magnetic photons. The electronic degrees of freedom can similarly be classified in terms of the angular momentum tensor or multipoles. Consequently, the interaction between electric multipoles is mediated by scalar photons, magnetic multipoles by magnetic photons, and magnetic toroidal multipoles by electric photons. These parameters can be integrated with the multiorbital Hubbard model or the Anderson lattice and serve as the foundational framework for investigating relativistic strongly correlated electron systems.

cond-mat.str-el↗

arXiv2023

Impurity effect on Bogoliubov Fermi surfaces: Analysis based on iron-based superconductors

The effect of impurities on a superconductor with Bogoliubov Fermi surfaces (BFSs) is studied using a realistic tight-binding model. Based on the band structure composed of $d$-orbitals in tetragonal FeSe, whose S-doped sample is a potential material for BFS, we construct the superconducting state by introducing a time-reversal broken pair potential in terms of the band index. We further consider the effect of impurities on the BFS, where the impurity potential is defined as a local potential for the original $d$-orbitals. The self-energy is calculated using the (self-consistent) Born approximation, which shows an enhancement of the single-particle spectral weight on the Fermi surface. This is consistent with the previous phenomenological theory and is justified by the present more detailed calculation based on the FeSe-based material.

cond-mat.supr-con↗

arXiv2023

Material-based analysis of spin-orbital Mott insulators

We present a framework for analyzing Mott insulators using a material-based tight-binding model. We start with a realistic multiorbital Hubbard model and derive an effective model for the localized electrons through the second-order perturbation theory with respect to intersite hopping. This effective model, known as the Kugel-Khomskii model, is described by SU($N$) generators, where $N$ is the number of localized states. We solve this model by the mean-field theory that takes local correlations into account and reveal spin-orbital ordered states. To include spatial correlations, we apply the classical Monte Carlo based on the path-integral approach with SU($N$) coherent states, and also derive the equation of motion for spin-orbital degrees of freedom. Our approach is applicable to any Mott insulator with reasonable computational cost. The $5d$-pyrochlore oxide is used here as demonstration.

cond-mat.str-el↗