arXiv subjects

Yuki Minami

Publications and source records attributed to Yuki Minami.

At least 19 recordsLinked to original sources

arXiv2026

Quantitative analysis of fluctuating hydrodynamics in uniform shear flow

Many theoretical predictions in fluctuating hydrodynamics under uniform shear flow have lacked precise quantitative verification because assessing the impact of analytical approximations is difficult and microscopic particle-based simulations have inherent limitations. To address this problem, we perform direct numerical simulations of the fluctuating Navier-Stokes equations with shear-periodic boundary conditions. We provide a decisive validation of two seminal frameworks: the Lutsko-Dufty theory for nonequilibrium long-range correlations, and the dynamic renormalization group (RG) theory pioneered by Forster, Nelson, and Stephen for anomalous transport. First, we demonstrate that the predictions of the Lutsko-Dufty theory are quantitatively valid from the viscous-dominated, short-wavelength regime to the shear-dominated, long-wavelength regime. Second, we test the quantitative predictive capability of the dynamic RG approach and show that the one-loop RG prediction is accurate even when the renormalization correction is comparable to the bare viscosity, a regime in which conventional perturbation theory fails. Our findings solidify the foundations of these classical theories, paving the way for quantitative analyses using fluctuating hydrodynamics.

cond-mat.stat-mech↗

arXiv2026

Unified geometric formalism for dissipation and its fluctuations in finite-time microscopic heat engines

Microscopic heat engines operate in regimes where thermodynamic quantities fluctuate strongly, making stochastic effects an essential aspect of their performance. However, existing geometric formulations of finite-time thermodynamics primarily characterize average dissipation and do not systematically capture its fluctuations. Here, we develop a unified geometric framework that consistently describes both the mean dissipated availability and its fluctuations. In the linear-response regime, we show that these quantities are governed by metric tensors constructed from equilibrium correlation functions, providing a common geometric structure for dissipation and its fluctuations. This framework yields geometric bounds on both the mean and variance of the dissipated availability, and thereby on the efficiency and its fluctuations. The formalism applies broadly to stochastic systems, including Markov jump processes and overdamped and underdamped Brownian dynamics, establishing a unified geometric description across microscopic heat engines.

cond-mat.stat-mech↗

arXiv2025

Dissipation anomaly in gradient-driven nonequilibrium steady states

Dissipation anomaly-the persistence of finite energy dissipation in the inviscid limit-is a hallmark of turbulence, sometimes regarded as the "zeroth law" of turbulent flows. Here, we demonstrate that this phenomenon is not exclusive to turbulence. Using fluctuating hydrodynamics, we show that a simple gradient-driven nonequilibrium steady state, in which a fluid is subjected to a constant scalar gradient but remains macroscopically quiescent, also exhibits dissipation anomaly. Direct numerical simulations and self-consistent mode-coupling theory reveal that the anomaly originates from giant, long-range nonequilibrium fluctuations amplified by the imposed gradient. While linear theory predicts a divergent dissipation in the inviscid limit, nonlinear mode coupling regularizes the divergence, yielding a finite anomalous dissipation. Our findings identify a new, non-turbulent arena for dissipation anomaly and establish the interplay between thermal noise and nonequilibrium driving as a fundamental route to singular behavior in hydrodynamics.

cond-mat.stat-mech↗

arXiv2025

Symmetry-based nonlinear fluctuating hydrodynamics in one dimension

We present a symmetry-based formulation of nonlinear fluctuating hydrodynamics (NFH) for one-dimensional many-particle systems with generic homogeneous nearest-neighbor interactions. We derive the hydrodynamic equations solely from symmetry and conservation principles, ensuring full consistency with thermalization. Using the dynamic renormalization group, we identify a KPZ-type fixed point, characterized by the dynamical exponent $z=3/2$ for both the sound and heat modes. Extensive numerical simulations of the derived NFH equations confirm this exponent and further reveal that both modes are close to the universal KPZ scaling function, the Prahofer-Spohn function. These findings establish a unified, symmetry-based framework for understanding universal transport and fluctuation phenomena in one-dimensional nonequili brium systems, independent of microscopic details.

cond-mat.stat-mech↗

arXiv2025

Looking at bare transport coefficients in fluctuating hydrodynamics

Bare transport coefficients in fluctuating hydrodynamics are not directly observable in bulk systems, as hydrodynamic fluctuations inevitably renormalize them into macroscopic values. In this work, we propose an operational method to determine the bare shear viscosity in two-dimensional dense fluids by focusing on fluid behavior near solid boundaries, where momentum scattering suppresses long-wavelength fluctuations. Using fluctuating hydrodynamic and molecular dynamics simulations supported by analytical arguments, we show that the viscosity measured near walls directly corresponds to the bare value. Based on this observation, we construct a practical protocol to extract the bare viscosity from microscopic data and verify its consistency by predicting flow profiles and equilibrium correlations. We further demonstrate that fluctuating hydrodynamics quantitatively reproduces fluid behavior down to atomic length scales.

cond-mat.stat-mech↗

arXiv2023

Topological Unwinding in an Exciton-Polariton Condensate Array

The phase distribution in a Bose-Einstein condensate can realize various topological states classified by distinct winding numbers. While states with different winding numbers are topologically protected in the linear Schr\"odinger equation, when nonlinearities are introduced, violations of the topological protection can occur, leading to unwinding. Exciton-polariton condensates constitute a nonlinear open-dissipative system that is well suited to studying such physics. Here we show that a one-dimensional array of exciton-polariton condensates displays a spontaneous phase unwinding from a $\pi$- to zero-state. We clarify that this collective mode transition is caused by the combined effect of nonlinearity and topological defects in the condensates. While the mode-switching phenomenon observed in our previous experiment was interpreted as the single-particle mode competition, we offer an alternative explanation in terms the collective phase unwinding and find its evidence by reanalyzing the experimental data. Our results open a route towards active control of the mode switching by manipulating the topological defects in prospective quantum polaritonic devices.

cond-mat.quant-gas↗

arXiv2022

Spontaneous symmetry breaking in two dimensions under nonequilibrium laminar flows

We study the long-range order in two dimensions where an order parameter is advected by laminar flows such as rotational, shear, and elongational flows. Under these flows, we analyze an ordered state of the $O(N)$ scalar model in the large-$N$ limit. We show that the stability of the ordered state depends on the flow pattern; shear and elongational flows stabilize the long-range order but rotational flow does not. We discuss the physical mechanism underlying our results by connecting static correlations of fluctuations and their dynamics based on the interaction representation used in quantum mechanics. We find that advective transport induces superdiffusion under shear and elongational flows, thereby stabilizing the long-range order.

cond-mat.stat-mech↗

arXiv2022

Effects of pairing gap and band gap on superfluid density in the inner crust of neutron stars

Calculations of the superfluid density in the inner crust of neutron stars by different approaches are in strong disagreement, which causes a debate on the accountability of pulsar glitches based on superfluidity. Taking a simple unified model, we study the dependence on approximation of the superfluid density in a periodic potential. In comparison with the Hartree-Fock-Bogoliubov (HF-Bogoliubov) theory which treats the effects of the band gap and the pairing gap on equal footing, we examine the HF-BCS-type approximation in which the former is incorporated in priority, and another approximation in which the latter is incorporated in priority. We find that, when the pairing gap and the band gap are comparable as in the inner crust of neutron stars, they need to be treated on equal footing, and the HF-BCS approximation can considerably underestimate the superfluid density even if the pairing gap is much smaller than the Fermi energy. Our result suggests that the validity of the HF-BCS approximation for evaluating the superfluid density in neutron star crusts is questionable.

nucl-th↗

arXiv2022

Relation between fluctuations and efficiency at maximum power for small heat engines

We study the ratio between the variances of work output and heat input, $\eta^{(2)}$, for a class of four-stroke heat engines which covers various typical cycles. Recent studies on the upper and lower bounds of $\eta^{(2)}$ are based on the quasistatic limit and the linear response regime, respectively. We extend these relations to the finite-time regime within the endoreversible approximation. We consider the ratio $\eta_{\text{MP}}^{(2)}$ at maximum power and find that the square of the Curzon-Ahlborn efficiency, $\eta_{\text{CA}}^2$, gives a good estimate of $\eta_{\text{MP}}^{(2)}$ for the class of heat engines considered, i.e., $\eta_{\text{MP}}^{(2)} \simeq \eta_{\text{CA}}^2$. This resembles the situation where the Curzon-Ahlborn efficiency gives a good estimate of the efficiency at maximum power for various kinds of finite-time heat engines. Taking an overdamped Brownian particle in a harmonic potential as an example, we can realize such endoreversible small heat engines and give an expression of the cumulants of work output and heat input. The approximate relation $\eta_{\text{MP}}^{(2)} \simeq \eta_{\text{CA}}^2$ is verified by numerical simulations. This relation also suggests a trade-off between the efficiency and the stability of finite-time heat engines at maximum power.

cond-mat.stat-mech↗

arXiv2022

Molecular dynamics study of shear-induced long-range correlations in simple fluids

We investigate long-range correlations (LRCs) induced by shear flow using the molecular dynamics (MD) simulation. We observe the LRCs by comparing the MD results with the linearized fluctuating hydrodynamics (LFH). We find that the MD result has large finite-size effects, and it prevents the occurrence of LRCs in small systems. We examine the finite-size effects using a sufficiently large system consisting of more than ten million particles, and verify the existence of shear-induced LRCs without ambiguity. Furthermore, we show that MD result is quantitatively consistent with the LFH solution for the large system. As we reduce the system size $L$ or increase the shear rate $\dot{\gamma}$, the hydrodynamic description gradually breaks down in the long-wavelength region. We define a characteristic wavenumber $k^{\rm vio}$ associated with the breakdown and find the nontrivial scaling relations $k^{\rm vio} \propto L^{-\omega}$ and $k^{\rm vio} \propto \dot{\gamma}$, where $\omega$ is an exponent depending on $\dot{\gamma}$. These relations enable us to estimate the finite-size effects in a larger-size simulation from a smaller system.

cond-mat.stat-mech↗

arXiv2021

Finite-Time Thermodynamics of Fluctuations in Microscopic Heat Engines

Fluctuations of thermodynamic quantities become non-negligible and play an important role when the system size is small. We develop finite-time thermodynamics of fluctuations in microscopic heat engines whose environmental temperature and mechanical parameter are driven periodically in time. Within the slow-driving regime, this formalism universally holds in a coarse-grained time scale whose resolution is much longer than the correlation time of the fluctuations, and is shown to be consistent with the relation analogous to the fluctuation-dissipation relation. Employing a geometric argument, a scenario to simultaneously minimize both the average and fluctuation of the dissipation in the Carnot cycle is identified. For this simultaneous optimization, the existence of a zero eigenvalue of the singular metric for the scale invariant equilibrium state is found to be essential. Furthermore, we demonstrate that our optimized protocol can improve the dissipation and its fluctuation over the current experiment.

cond-mat.stat-mech↗

arXiv2021

Emergence of surface long-range order under uniform shear flow

We study the two-dimensional surface long-range order in a non-equilibrium steady state under shear flow using the three-dimensional conserved $O(N)$ model. Whereas the correlation on the surface is enhanced by increasing interactions within the surface, the long-range order cannot be realized at equilibrium because of divergent thermal fluctuations associated with the low dimensionality of the surface. Here, the shear flow is applied parallel to the surface, on which the flow is set to zero. Despite the shear flow not affecting the order parameter on the surface directly, the fluctuations at the surface are strongly suppressed by the flow away from the surface, leading to the surface long-range order. We demonstrate these results through an exact analysis in the large-$N$ limit, where non-linear fluctuations are self-consistently treated.

cond-mat.stat-mech↗

arXiv2020

Long-range phase order in two dimensions under shear flow

We theoretically and numerically investigate a two-dimensional O(2) model where an order parameter is convected by shear flow. We show that a long-range phase order emerges in two dimensions as a result of anomalous suppression of phase fluctuations by the shear flow. Furthermore, we use the finite-size scaling theory to demonstrate that a phase transition to the long-range ordered state from the disordered state is second order. At a transition point far from equilibrium, the critical exponents turn out to be close to the mean-field value for equilibrium systems.

cond-mat.stat-mech↗

arXiv2020

Rainbow Nambu-Goldstone modes under a shear flow

We study an $O(N)$ scalar model under shear flow and its Nambu-Goldstone modes associated with spontaneous symmetry breaking $O(N) \to O(N-1)$. We find that the Nambu-Goldstone mode splits into an infinite number of gapless modes, which we call the rainbow Nambu-Goldstone modes. They have different group velocities and the fractional dispersion relation $\omega \sim k_1^{2/3}$, where $k_1$ is the wavenumber along the flow. Such behaviors do not have counterparts in an equilibrium state.

cond-mat.stat-mech↗

arXiv2020

Spontaneous symmetry breaking and Nambu-Goldstone modes in open classical and quantum systems

We discuss spontaneous symmetry breaking of open classical and quantum systems. When a continuous symmetry is spontaneously broken in an open system, a gapless excitation mode appears corresponding to the Nambu-Goldstone mode. Unlike isolated systems, the gapless mode is not always a propagation mode, but it is a diffusion one. Using the Ward-Takahashi identity and the effective action formalism, we establish the Nambu-Goldstone theorem in open systems, and derive the low-energy coefficients that determine the dispersion relation of Nambu-Goldstone modes. Using these coefficients, we classify the Nambu-Goldstone modes into four types: type-A propagation, type-A diffusion, type-B propagation, and type-B diffusion modes.

hep-th↗

arXiv2019

Thermodynamic entropy as a Noether invariant in a Langevin equation

We study the thermodynamic entropy as a Noether invariant in a stochastic process. Following the Onsager theory, we consider the Langevin equation for a thermodynamic variable in a thermally isolated system. By analyzing the Martin-Siggia-Rose-Janssen-de Dominicis action of the Langevin equation, we find that this action possesses a continuous symmetry in quasi-static processes, which leads to the thermodynamic entropy as the Noether invariant for the symmetry.

cond-mat.stat-mech↗

arXiv2018

Relativistic hydrodynamics from projection operator method

We study relativistic hydrodynamics in the linear regime, based on Mori's projection operator method. In relativistic hydrodynamics, it is considered that ambiguity about the fluid velocity occurs from a choice of a local rest frame: the Landau and Eckart frames. We find that the difference of the frames is not the choice of the local rest frame, but rather that of dynamic variables in the linear regime. We derive hydrodynamic equations in the both frames by the projection operator method. We show that natural derivation gives the linearized Landau equation. Also, we find that, even for the Eckart frame, the slow dynamics is actually described by the dynamic variables for the Landau frame.

hep-ph↗

arXiv2018

Spontaneous symmetry breaking and Nambu-Goldstone modes in dissipative systems

We discuss spontaneous breaking of internal symmetry and its Nambu-Goldstone (NG) modes in dissipative systems. We find that there exist two types of NG modes in dissipative systems corresponding to type-A and type-B NG modes in Hamiltonian systems. To demonstrate the symmetry breaking, we consider a $O(N)$ scalar model obeying a Fokker-Planck equation. We show that the type-A NG modes in the dissipative system are diffusive modes, while they are propagating modes in Hamiltonian systems. We point out that this difference is caused by the existence of two types of Noether charges, $Q_R^α$ and $Q_A^α$: $Q_R^α$ are symmetry generators of Hamiltonian systems, which are not conserved in dissipative systems. $Q_A^α$ are symmetry generators of dissipative systems described by the Fokker-Planck equation, which are conserved. We find that the NG modes are propagating modes if $Q_R^α$ are conserved, while those are diffusive modes if they are not conserved. We also consider a $SU(2)\times U(1)$ scalar model with a chemical potential to discuss the type-B NG modes. We show that the type-B NG modes have a different dispersion relation from those in the Hamiltonian systems.

cond-mat.stat-mech↗