arXiv subjects

Takumi Sato

Publications and source records attributed to Takumi Sato.

At least 19 recordsLinked to original sources

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

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

Forcing a unique minimum spanning tree and a unique shortest path

A forcing set $S$ in a combinatorial problem is a set of elements such that there is a unique solution that contains all the elements in $S$. An anti-forcing set is the symmetric concept: a set $S$ of elements is called an anti-forcing set if there is a unique solution disjoint from $S$. There are extensive studies on the computational complexity of finding a minimum forcing set in various combinatorial problems, and the known results indicate that many problems are harder than their classical counterparts: the decision version of finding a minimum forcing set for perfect matchings is NP-complete [Adams et al., Discrete Mathematics, 2004], and that of finding a minimum forcing set for satisfying assignments for 3CNF formulas is $Σ_2^P$-complete [Hatami-Maserrat, Discrete Applied Mathematics, 2005]. In this paper, we investigate the complexity of finding minimum forcing and anti-forcing sets for the shortest $s$-$t$ path problem and the minimum-weight spanning tree problem. We show that, unlike the aforementioned results, these problems are tractable, with the exception of the decision version of finding a minimum anti-forcing set for shortest $s$-$t$ paths, which is NP-complete. To complement this intractability result, we design fixed-parameter tractable algorithms for finding a minimum anti-forcing set for shortest $s$-$t$ paths.

cs.DS↗

arXiv2026

Analysis of molecular dynamics simulation data via statistical distances between covariance matrices

Molecular dynamics (MD) simulations are powerful tools for elucidating the macroscopic physical properties of materials from microscopic atomic behaviors. However, the massive, high-dimensional datasets generated by MD simulations pose a significant challenge for analysis, necessitating efficient dimensionality reduction and feature extraction techniques. While existing methods such as principal component analysis and unsupervised learning have been utilized, issues regarding data efficiency and computational cost remain. In this study, we propose a statistical analysis framework focusing on the analysis of the particle data distributions through their covariance matrices, corresponding to the second-order moments of MD trajectory data. Discrepancies between system states are quantified using statistical distances between these covariance matrices. By applying dimensionality reduction to the resulting distance matrix, we extract lower-dimensional features that characterize the systems' dynamics. We validate the proposed method using Lennard-Jones (LJ) particle systems under different temperature conditions, as well as separate bulk systems of ice and liquid water. The results of LJ particles demonstrate an approximately linear correlation between the first principal component obtained through dimensionality reduction of the distance matrix and the diffusion coefficient. This suggests that global physical properties can be effectively inferred from local statistical information, such as covariance matrices, offering a data-efficient alternative for analyzing complex molecular systems. Furthermore, in the case of separate bulk systems of ice and liquid water, the method successfully distinguishes between the two phases, highlighting its potential for characterizing phase transitions and structural differences in molecular systems.

stat.AP↗

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

Orbital magnetic octupole in crystalline solids and characterization of orbital altermagnetism

Magnetic multipole moments beyond dipoles have emerged as key descriptors of unconventional electromagnetic responses in crystalline solids. However, a gauge-invariant bulk expression for orbital magnetic multipole moments has remained elusive, hindering a unified understanding of their physical consequences. Here we formulate a gauge-invariant expression for the orbital magnetic octupole moment in periodic crystals and investigate its behavior in two models with distinct origins of magnetism: a spinless two-orbital model with orbital magnetic order arising from complex hopping and a two-sublattice $d$-wave altermagnetic model based on antiferromagnetic spin order. We also show that the orbital magnetic octupole is naturally linked to a higher-rank Hall response induced by spatially nonuniform electric fields, leading to a generalized Středa-type relation. Our results further demonstrate that the orbital magnetic octupole provides a quantitative characterization of \textsl{orbital altermagnetism}.

cond-mat.mes-hall↗

arXiv2026

Finite-momentum superconducting states due to odd-frequency Cooper pairing correlations

This paper discusses the origin of a nonuniform superconducting state in which Cooper pairs have a small but finite center-of-mass momentum. We analyze the instability of the normal state to such finite-momentum states using the pole of the pair fluctuation propagator in weak-coupling superconductors. The finite-momentum superconducting state is realized when the odd-frequency pairing correlations in the uniform superconducting state are expected to have sufficiently large amplitudes. We provide a perspective for a comprehensive understanding of inhomogeneous superconductivity and related phenomena.

cond-mat.supr-con↗

arXiv2026

Direct loading of a Sr magneto-optical trap from a thermal atomic beam

We demonstrate direct loading of a strontium (Sr) magneto-optical trap (MOT) from a thermal atomic beam in a single-chamber vacuum system. The MOT operates without a Zeeman slower, a slowing laser, a two-dimensional MOT, or differential pumping, while the entire system is maintained in the ultra-high-vacuum regime by a single ion pump. At an oven temperature of $395\,\mathrm{{}^\circ C}$, the MOT captures up to $10^{7}$ ${}^{88}\mathrm{Sr}$ atoms with a loading rate of $10^{7}\,\mathrm{atoms\,s^{-1}}$, while sustaining a background gas pressure of $1 \times 10^{-9} \,\mathrm{Torr}$. At this oven temperature, the MOT lifetime limited by collisions with background gas is $\sim 5 \,\mathrm{s}$, with the atom number primarily constrained by light-assisted two-body collisions. Eliminating differential pumping and precooling stages significantly reduces the system's size, weight, and power requirements, providing a robust and practical platform for field-deployable and spaceborne optical lattice clocks, as well as a variety of other applications requiring compact ultracold atom sources.

physics.atom-ph↗

arXiv2026

Edge element DtN method for electromagnetic scattering poles of perfectly conducting obstacles

Meromorphic continuation of the scattering operator leads to scattering poles (resonances) in the complex plane. Despite their significance, numerical investigation of scattering poles remains limited. In this paper, we propose and analyze a numerical method to compute electromagnetic poles of perfectly conducting obstacles. The unbounded domain for the scattering problem is truncated using the DtN mapping and the poles are shown to be the eigenvalues of a holomorphic Fredholm operator function related to Maxwell's equations. Edge elements are used for discretization. The convergence is proved using the abstract approximation theory for eigenvalue problems of holomorphic Fredholm operator functions. The proposed finite element DtN approach is free of non-physical poles. A spectral indicator method is then employed to compute the resulting nonlinear matrix eigenvalue problem. Numerical examples are presented to demonstrate the effectiveness of the method.

math.NA↗

arXiv2026

Quantum theory of magnetic octupole in periodic crystals and application to $d$-wave altermagnets

Magnetic multipoles have been recognized as order parameters characterizing magnetic structure in solids. Recently, magnetic octupoles have been proposed as the order parameters of time-reversal-symmetry breaking centrosymmetric antiferromagnets exhibiting nonrelativistic spin splitting, which is referred to as ``altermagnet''. However, a gauge-invariant formulation of magnetic octupoles in crystalline solids remains elusive. Here, we present a gauge-invariant expression of spin magnetic octupoles in periodic crystals based on quantum mechanics and thermodynamics, which can be used to quantitatively characterize time-reversal-symmetry breaking antiferromagnets including $d$-wave altermagnets. The allowed physical response tensors are classified beyond symmetry considerations, and direct relationships are established for some of them in insulators at zero temperature. Furthermore, our expression reveals a contribution from an anisotropic magnetic dipole, which has the same symmetry as conventional spin and orbital magnetic dipoles but carries no net magnetization. We discuss the relation between the anisotropic magnetic dipole and the anomalous Hall effect.

cond-mat.mes-hall↗

arXiv2025

Enhanced premelting at the ice-rubber interface using all-atom molecular dynamics simulation

The ice-rubber interface is critical in applications such as tires and shoe outsoles, yet its molecular tribology remains unclear. Using all-atom molecular dynamics simulations, we studied premelting layers at the basal face of ice in contact with styrene-butadiene rubber from 254 to 269 K. Despite its hydrophobicity, rubber enhances structural disorder of interfacial water, promoting premelting. In contrast, water mobility is suppressed by confinement from polymer chains, leading to glassy dynamics distinct from the ice-vapor interface. Near the melting point, rubber chains become more flexible and penetrate the premelting layer, forming a mixed rubber-water region that couples the dynamics of both components. These results suggest that nanoscale roughness and morphology of hydrophobic polymers disrupt ice hydrogen-bond networks, thereby enhancing premelting. Our findings provide molecular-level insight into ice slipperiness and inform the design of polymer materials with controlled ice adhesion and friction.

cond-mat.soft↗

arXiv2025

Molecular Dynamics Investigation of Static and Dynamic Interfacial Properties in Ice-Polymer Premelting Layers

Premelting at the ice-polymer interfaces, in which a quasi-liquid layer (QLL) forms below the melting point, is strongly influenced by polymer surface chemistry; however, the molecular-scale mechanisms underlying these effects remain poorly understood. This study employs large-scale molecular dynamics simulations combined with machine learning-assisted analysis to elucidate how polymer type (hydrophilic vs hydrophobic) modulates interfacial premelting. Our simulations reveal that hydrophilic and hydrophobic polymer surfaces have distinct effects on the QLL thickness, interfacial water structure, and diffusivity. Specifically, a hydrophilic polymer interface promotes a thicker QLL with more ordered interfacial water and lower diffusivity, whereas a hydrophobic interface induces a thinner QLL with a less ordered interfacial water structure and higher diffusivity. These results advance the understanding of polymer-mediated interfacial melting phenomena and offer guidance for designing anti-icing and low-friction materials.

cond-mat.soft↗

arXiv2025

FEM-DtN-SIM Method for Computing Resonances of Schrödinger Operators

The study of resonances of the Schrödinger operator has a long-standing tradition in mathematical physics. Extensive theoretical investigations have explored the proximity of resonances to the real axis, their distribution, and bounds on the counting functions. However, computational results beyond one dimension remain scarce due to the nonlinearity of the problem and the unbounded nature of the domain. We propose a novel approach that integrates finite elements, Dirichlet-to-Neumann (DtN) mapping, and the spectral indicator method. The DtN mapping, imposed on the boundary of a truncated computational domain, enforces the outgoing condition. Finite elements allow for the efficient handling of complicated potential functions. The spectral indicator method effectively computes (complex) eigenvalues of the resulting nonlinear algebraic system without introducing spectral pollution. The viability of this approach is demonstrated through a range of numerical examples.

math.NA↗

arXiv2025

Thermoelectric effect in a superconductor with Bogoliubov Fermi surfaces

We study theoretically the thermoelectric effect in a superconducting state having the Bogoliubov-Fermi surfaces which stays in a thin superconducting layer between a conventional superconductor and an insulator. The thermoelectric coefficients calculated based on the linear response theory show the remarkable anisotropy in real space, which are explained well by the anisotropic shape of the Bogoliubov-Fermi surface in momentum space. Our results indicate a way to check the existence of the Bogoliubov-Fermi surfaces in a stable superconducting state because the anisotropy is controlled by the direction of an applied magnetic field.

cond-mat.supr-con↗

arXiv2025

Investigating the hyperparameter space of deep neural network models for reaction coordinates

Identifying reaction coordinates (RCs) is a key to understanding the mechanism of reactions in complex systems. Deep neural network (DNN) and machine learning approaches have become a powerful tool to find the RC. On the other hand, the hyperparameters that determine the DNN model structure can be highly flexible and are often selected intuitively and in a non-trivial and tedious manner. Furthermore, how the hyperparameter choice affects the RC quality remains obscure. Here, we explore the hyperparameter space by developing the hyperparameter tuning approach for the DNN model for RC and investigate how the parameter set affects the RC quality. The DNN model is built to predict the committor along the RC from various collective variables by minimizing the cross-entropy function; the hyperparameters are automatically determined using the Bayesian optimization method. The approach is applied to study the isomerization of alanine dipeptide in vacuum and in water, and the features that characterize the RC are extracted using the explainable AI (XAI) tools. The results show that the DNN models with diverse structures can describe the RC with similar accuracy, and furthermore, the features analyzed by XAI are highly similar. This indicates that the hyperparameter space is multimodal. The electrostatic potential from the solvent to the hydrogen H18 plays an important role in the RC in water. The current study shows that the structure of the DNN models can be rather flexible, while the suitably optimized models share the same features; therefore, a common mechanism from the RC can be extracted.

physics.chem-ph↗

arXiv2024

Discontinuous Transition to Superconducting Phase

We discuss the instability of uniform superconducting states that contain the pairing correlations belonging to the odd-frequency symmetry class. The instability originates from the paramagnetic response of odd-frequency Cooper pairs and is considerable at finite temperatures. As a result, the pair potential varies discontinuously at the transition temperature when the amplitude of the odd-frequency pairing correlation functions is sufficiently large. The discontinuous transition to the superconducting phase is a general feature of superconductors that include odd-frequency Cooper pairs.

cond-mat.supr-con↗

arXiv2023

Fulde-Ferrell-Larkin-Ovchinnikov state in a superconducting thin film attached to a ferromagnetic cluster

We study theoretically the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states appearing locally in a superconducting thin film with a small circular magnetic cluster. The pair potential, the pairing correlations, the free-energy density, and the quasiparticle density of states are calculated for several cluster sizes and the exchange potentials by solving the Eilenberger equation in two dimensions. The number of nodes in the pair potential increases with increasing the exchange potential and cluster size. The local FFLO states are stabilized by the superconducting condensate away from the magnetic cluster even though the free-energy density beneath the ferromagnet exceeds locally the normal-state value. The analysis of the pairing-correlation functions shows that the spatial variation of the spin-singlet $s$-wave pair potential generates $p$-wave Cooper pairs, and that odd-frequency Cooper pairs govern the inhomogeneous subgap spectra in the local density of states. We also discuss a way of detecting the local FFLO states based on the calculated quasiparticle density of states.

cond-mat.supr-con↗

arXiv2022

Spin Susceptibility of a J=3/2 Superconductor

We discuss the spin susceptibility of superconductors in which a Cooper pair consists of two electrons having the angular momentum J=3/2 due to strong spin-orbit interactions. The susceptibility is calculated analytically for pseudospin quintet states in a cubic superconductor within the linear response to a Zeeman field. The susceptibility for $A_{1g}$ symmetry states is isotropic in real space. For $E_g$ and $T_{2g}$ symmetry cases, the results depend sensitively on choices of order parameter. The susceptibility is isotropic for a $T_{2g}$ symmetry state, whereas it becomes anisotropic for an $E_{g} $ symmetry state. We also find in a $T_{2g}$ state that the susceptibility tensor has off-diagonal elements.

cond-mat.supr-con↗