arXiv subjects

Yuta Sekino

Publications and source records attributed to Yuta Sekino.

17 recordsLinked to original sources

arXiv2026

Programmable Spin Conversion in Gradient Quantum Matter

We propose programmable spin conversion in ultracold gases as gradient quantum matter, whose spin-dependent self-energy varies in space. Quantum kinetic theory shows that a dissipative self-energy curvature turns a force-driven scalar anisotropy into a spin source with mixed longitudinal-transverse momentum parity. Spin-resolved time-of-flight imaging can reveal a transverse spin texture that changes sign when either the drive or programmed curvature is reversed. Ultracold gases thereby offer a controllable spin source for gradient quantum matter.

cond-mat.quant-gas↗

arXiv2026

Thermal Spin Polarization Driven by Nuclear Spin-Orbit Coupling in Neutron Star Pasta

We discuss anomalous spin polarization on the surface of nuclear pasta in a neutron star, driven by a nuclear spin-orbit interaction. We present an effective two-band model of surface-localized neutrons near the nuclear pasta. The central point is the emergence of a Rashba-type spin-orbit hybridization generated by the neutron--nucleus spin-orbit force in the presence of the strong density gradient normal to the pasta surface. Starting from a single-particle Hamiltonian with a central potential and a standard nuclear spin-orbit interaction, we show that the surface spin polarization occurs due to the thermal inhomogeneity even in the absence of a magnetic field. Our study links neutron-star physics and solid-state spintronics and would contribute to understanding the interplay between spin dynamics and strong magnetic fields.

nucl-th↗

arXiv2025

Gyromagnetic Angular Momentum Interconversion in Neutron Stars

We propose a novel mechanism for angular momentum (AM) exchange between the crust and core of a neutron star (NS) via the gyromagnetic effect. Using extended hydrodynamics, we model the star by incorporating macroscopic AM and microscopic AM originating from neutron orbital and spin AM. We reveal that macroscopic dynamics in the crust can inform microscopic AM in the core leading to neutron spin polarization, and offer alternative scenario of (anti-)glitches. This work highlights the overlooked multi-scale AM interconversions in NS physics, paving the way for gyromagnetic astrophysics.

astro-ph.HE↗

arXiv2025

Warm Starting of CMA-ES for Contextual Optimization Problems

Several practical applications of evolutionary computation possess objective functions that receive the design variables and externally given parameters. Such problems are termed contextual optimization problems. These problems require finding the optimal solutions corresponding to the given context vectors. Existing contextual optimization methods train a policy model to predict the optimal solution from context vectors. However, the performance of such models is limited by their representation ability. By contrast, warm starting methods have been used to initialize evolutionary algorithms on a given problem using the optimization results on similar problems. Because warm starting methods do not consider the context vectors, their performances can be improved on contextual optimization problems. Herein, we propose a covariance matrix adaptation evolution strategy with contextual warm starting (CMA-ES-CWS) to efficiently optimize the contextual optimization problem with a given context vector. The CMA-ES-CWS utilizes the optimization results of past context vectors to train the multivariate Gaussian process regression. Subsequently, the CMA-ES-CWS performs warm starting for a given context vector by initializing the search distribution using posterior distribution of the Gaussian process regression. The results of the numerical simulation suggest that CMA-ES-CWS outperforms the existing contextual optimization and warm starting methods.

cs.NE↗

arXiv2025

Thermomagnetic anomalies in quantum magnon transport caused by tunable junction geometries in cold atomic systems

We study magnon-driven spin and heat transport in a magnetic linear junction (MLJ) formed by two ferromagnets in optical lattices linked via linearly aligned bonds. Using the Schwinger-Keldysh formalism, we uncover that under weak effective Zeeman fields, where Bose-Einstein statistics of magnons dominate, magnonic criticality dramatically enhances spin and thermal conductances. These singular transport properties depend on the junction geometry, and the transport properties qualitatively differ between the linear junction in this study and the point contact in our previous work. The quantum-enhanced conductances result in the breakdown of the magnonic Wiedemann-Franz (WF) law. In the classical regime at temperatures much lower than magnon energy gaps, we find that a magnonic Lorenz number becomes independent of temperature yet dependent on junction geometry, sharply contrasting with the universal WF law for Fermi liquids. We also find that the interface geometry of MLJ decouples spin and heat relaxations between ferromagnets with decay times insensitive to temperature and effective Zeeman fields. These dynamics reveal junction-geometry-sensitive magnon transport distinct from Fermi liquids, paving the way for new avenues in thermomagnetic research leveraging the tunability of cold atomic systems.

cond-mat.quant-gas↗

arXiv2024

Thermomagnetic Anomalies by Magnonic Criticality in Ultracold Atomic Transport

We investigate thermomagnetic transport in an ultracold atomic system with two ferromagnets linked via a magnetic quantum point contact. Using nonequilibrium Green's function approach, we show a divergence in spin conductance and a slowing down of spin relaxation that manifest in the weak effective-Zeeman-field limit. These anomalous spin dynamics result from the magnonic critical point at which magnons become gapless due to spontaneous magnetization. Our findings unveil untapped dynamics in ultracold atomic systems, opening new avenues in thermomagnetism.

cond-mat.quant-gas↗

arXiv2023

Non-Hermitian $p$-wave superfluid and effects of the inelastic three-body loss in a one-dimensional spin-polarized Fermi gas

We theoretically investigate non-Hermitian $p$-wave Fermi superfluidity in one-dimensional spin-polarized Fermi gases which is relevant to recent ultracold atomic experiments. Considering an imaginary atom-dimer coupling responsible for the three-body recombination process in the Lindblad formalism, we discuss the stability of the superfluid state against the atomic loss effect. Within the two-channel non-Hermitian BCS-Leggett theory, the atomic loss is characterized by the product of the imaginary atom-dimer coupling and the $p$-wave effective range. Our results indicate that for a given imaginary atom-dimer coupling, a smaller magnitude of the effective ranges of $p$-wave interaction is crucial for reaching the non-Hermitian $p$-wave Fermi superfluid state.

cond-mat.quant-gas↗

arXiv2023

Exploring $^3P_0$ Superfluid in Dilute Spin-Polarized Neutron Matter

We explore the theoretical possibility of $^3P_0$ neutron superfluid in dilute spin-polarized neutron matter, which may be relevant to the crust region of a magnetized neutron star. In such a dilute regime where the neutron Fermi energy is less than 1 MeV, the $^1S_0$ neutron superfluid can be suppressed by a strong magnetic field of the compact star. In the low-energy limit relevant for dilute neutron matter, the $^3P_0$ interaction is stronger than the $^3P_2$ one which is believed to induce the triplet superfluid in the core. We present the ground-state phase diagram of dilute neutron matter with respect to the magnetic field and numerically estimate the critical temperature of the $^3P_0$ neutron superfluid, which is found to exceed $10^7$ K.

nucl-th↗

arXiv2023

Dominant Andreev Reflection through Nonlinear Radio-Frequency Transport

We theoretically propose the laser-induced Andreev reflection between two-component Fermi superfluid and normal states via spatially-uniform Rabi couplings. By analyzing the tunneling current between the superfluid and normal states up to the fourth order in the Rabi couplings, we find that the Andreev current exhibits unconventional non-Ohmic transport at zero temperature. Remarkably, the Andreev current gives the only contribution in the synthetic junction system at zero detunings regardless of the ratio of the chemical potential bias to the superfluid gap, which is in sharp contrast to that in the conventional superconductor-normal metal junction. Our result may also pave a way for understanding the black hole information paradox through the Andreev reflection as a quantum-information mirror.

cond-mat.quant-gas↗

arXiv2023

Spin conductivity spectrum and spin superfluidity in a binary Bose mixture

We investigate the spectrum of spin conductivity for a miscible two-component Bose-Einstein condensate (BEC) that exhibits spin superfluidity. By using the Bogoliubov theory, the regular part being the spin conductivity at finite ac frequency and the spin Drude weight characterizing the delta-function peak at zero frequency are analytically computed. We demonstrate that the spectrum exhibits a power-law behavior at low frequency, reflecting gapless density and spin modes specific to the binary BEC. At the phase transition points into immiscible and quantum-droplet states, the change in quasiparticle dispersion relations modifies the power law. In addition, the spin Drude weight becomes finite, indicating zero spin resistivity due to spin superfluidity. Our results also suggest that the Andreev-Bashkin drag density is accessible by measuring the spin conductivity spectrum.

cond-mat.quant-gas↗

arXiv2022

Non-Hermitian topological Fermi superfluid near the $p$-wave unitary limit

We theoretically discuss the non-Hermitian superfluid phase transition in one-dimensional two-component Fermi gases near the $p$-wave Feshbach resonance accompanied by the two-body loss associated with the dipolar relaxation. For the first time we point out that this system gives us an opportunity to explore the interplay among various non-trivial properties such as universal thermodynamics at divergent $p$-wave scattering length, topological phase transition at vanishing chemical potential, and non-Hermitian Bardeen-Cooper-Schrieffer(BCS) to Bose-Einstein condensate (BEC) transition, in a unified manner. In the BCS phase, the loss-induced superfluid-normal transition occurs when the exceptional point appears in the effective non-Hermitian Hamiltonian. In the BEC phase, the diffusive gapless mode can be regarded as a precursor of the instability of the superfluid state. Moreover, we show that the superfluid state is fragile against the two-body loss near the topological phase transition point.

cond-mat.quant-gas↗

arXiv2022

Optical spin conductivity in ultracold quantum gases

We show that the optical spin conductivity being a small AC response of a bulk spin current and elusive in condensed matter systems can be measured in ultracold atoms. We demonstrate that this conductivity contains rich information on quantum states by analyzing experimentally achievable systems such as a spin-1/2 superfluid Fermi gas, a spin-1 Bose-Einstein condensate, and a Tomonaga-Luttinger liquid. The obtained conductivity spectra being absent in the Drude conductivity reflect quasiparticle excitations and non-Fermi liquid properties. Accessible physical quantities include the superfluid gap and the contact for the superfluid Fermi gas, gapped and gapless spin excitations as well as quantum depletion for the Bose-Einstein condensate, and the spin part of the Tomonaga-Luttinger liquid parameter elusive in cold-atom experiments. Unlike its mass transport counterpart, the spin conductivity serves as a probe applicable to clean atomic gases without disorder and lattice potentials. Our formalism can be generalized to various systems such as spin-orbit coupled and nonequilibrium systems.

cond-mat.quant-gas↗

arXiv2022

Optical spin transport theory of spin-1/2 topological Fermi superfluids

We theoretically investigate optical (frequency-dependent) bulk spin transport properties in a spin-1/2 topological Fermi superfluid. We specifically consider a one-dimensional system with an interspin {\it p}-wave interaction, which can be realized in ultracold atom experiments. Developing the BCS-Leggett theory to describe the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensate (BEC) evolution and the $\mathbb{Z}_2$ topological phase transition in this system, we show how the spin transport reflects these many-body aspects. We find that the optical spin conductivity, which is a small AC response of a spin current, shows the spin gapped spectrum in the wide parameter region and the gap closes at $\mathbb{Z}_2$ topological phase transition point. Moreover, the validity of the low-energy effective model of the Majorana zero mode is discussed along the BCS-BEC evolution in connection with the scale invariance at {\it p}-wave unitarity.

cond-mat.quant-gas↗

arXiv2021

Field-theoretical aspects of one-dimensional Bose and Fermi gases with contact interactions

We investigate local quantum field theories for one-dimensional (1D) Bose and Fermi gases with contact interactions, which are closely connected with each other by Girardeau's Bose-Fermi mapping. While the Lagrangian for bosons includes only a two-body interaction, a marginally relevant three-body interaction term is found to be necessary for fermions. Because of this three-body coupling, the three-body contact characterizing a local triad correlation appears in the energy relation for fermions, which is one of the sum rules for a momentum distribution. In addition, we apply in both systems the operator product expansion to derive large-energy and momentum asymptotics of a dynamic structure factor and a single-particle spectral density. These behaviors are universal in the sense that they hold for any 1D scattering length at any temperature. The asymptotics for the Tonks-Girardeau gas, which is a Bose gas with a hardcore repulsion, as well as the Bose-Fermi correspondence in the presence of three-body attractions are also discussed.

cond-mat.quant-gas↗

arXiv2020

Mesoscopic spin transport between strongly interacting Fermi gases

We investigate a mesoscopic spin current for strongly interacting Fermi gases through a quantum point contact. Under the situation where spin polarizations in left and right reservoirs are same in magnitude but opposite in sign, we calculate the contribution of quasiparticles to the current by means of the linear response theory and many-body $T$-matrix approximation. For a small spin-bias regime, the current in the vicinity of the superfluid transition temperature is strongly suppressed due to the formation of pseudogaps. For a large spin-bias regime where the gases become highly polarized, on the other hand, the current is affected by the enhancement of a minority density of states due to Fermi polarons. We also discuss the broadening of a quasiparticle peak associated with an attractive polaron at a large momentum, which is relevant to the enhancement.

cond-mat.quant-gas↗

arXiv2018

Quantum droplet of one-dimensional bosons with a three-body attraction

Ultracold atoms offer valuable opportunities where interparticle interactions can be controlled at will. In particular, by extinguishing the two-body interaction, one can realize unique systems governed by the three-body interaction, which is otherwise hidden behind the two-body interaction. Here we study one-dimensional bosons with a weak three-body attraction and show that they form few-body bound states as well as a many-body droplet stabilized by the quantum mechanical effect. Their binding energies relative to that of three bosons are all universal and the ground-state energy of the dilute droplet is found to grow exponentially as $E_N/E_3\to\exp(8N^2/\sqrt3π)$ with increasing particle number $N\gg1$. The realization of our system with coupled two-component bosons in an optical lattice is also discussed.

cond-mat.quant-gas↗

arXiv2018

Comparative study of one-dimensional Bose and Fermi gases with contact interactions from the viewpoint of universal relations for correlation functions

One-dimensional spinless Bose and Fermi gases with contact interactions have the close interrelation via Girardeau's Bose-Fermi mapping, leading to the correspondences in their energy spectra and thermodynamics. However, correlation functions are in general not identical between these systems. We derive in both systems the exact universal relations for correlation functions, which hold for any energy eigenstate and any statistical ensemble of the eigenstates with or without a trapping potential. These relations include the large-momentum behaviors of static structure factors and of momentum distributions as well as energy relations, which connect the sums of kinetic and interaction energies to the momentum distributions. The relations involve two- and three-body contacts, which are the integrals of local pair and triad correlations, respectively. We clarify how the relations for bosons and fermions differ and are connected with each other. In particular, we find that the three-body contact makes no contribution to the bosonic energy relation, but it plays a crucial role in the fermionic one. In addition, we compute the exact momentum distribution for any number of fermions in the unitary limit.

cond-mat.quant-gas↗