arXiv subjects

Yuki Izumida

Publications and source records attributed to Yuki Izumida.

At least 19 recordsLinked to original sources

arXiv2026

Irreversibility of many-body harmonic oscillators characterized by generalized Husimi's adiabaticity parameter

In a classical paper [K. Husimi, Prog. Theor. Phys. 9, 381 (1953)], Husimi showed that, for a single harmonic oscillator with a time-dependent angular frequency and initial conditions sampled from an equilibrium distribution, the averaged energy cannot decrease after a cyclic operation. This irreversibility is quantified by Husimi's adiabaticity parameter constituted with adiabatic invariants. In this work, we generalize Husimi's framework to a many-body system of one-dimensional harmonic oscillators coupled on an arbitrary connected network, with all spring constants sharing a common time dependence. For a system attached to a fixed wall, the dynamics can be decomposed into independent normal modes by transforming to mass-weighted coordinates and diagonalizing the resulting positive definite matrix, with each mode characterized by its own Husimi's adiabaticity parameter. By defining the generalized Husimi's adiabaticity parameter as their arithmetic mean, we derive the exact time evolution of the averaged energy and establish its non-decrease under cyclic operations. Numerical simulations confirm these results for both a uniform nearest-neighbor chain and a heterogeneous network.

cond-mat.stat-mech↗

arXiv2026

Universal Thermodynamic Law Governing Stochastic Pendulum Clocks

Clocks' performance, especially their precision, is fundamentally limited by thermodynamic laws as they are physical devices. Yet, it is known that the established thermodynamic uncertainty relation (TUR), which imposes an upper bound on the uncertainty product of the precision of oscillations and entropy production, is violated for underdamped systems such as stochastic pendulum clocks. Here, we show that for a general class of stochastic pendulum clocks described as a weakly nonlinear oscillator the uncertainty product of the phase of a pendulum clock and entropy production takes a universal and simple form that depends solely on the degree of nonlinearity. Its validity and limitations are examined using several representative models of pendulum clocks. Our framework reveals a universal thermodynamic principle governing stochastic pendulum clocks beyond the conventional TUR and provides a foundation for designing optimal pendulum clocks that operate efficiently in stochastic environments.

cond-mat.stat-mech↗

arXiv2026

Nonlinear refrigerator with a finite-sized cold heat bath

We study the refrigerator working between a finite-sized cold heat bath and an infinite-sized hot heat bath (environment) in the nonlinear response regime. We assume that the initial temperature $T_i$ of the finite-sized cold heat bath satisfies $T_i\leq T_h$, where $T_h$ is the temperature of the hot heat bath. By consuming the input power, the refrigerator transfers the heat from a finite-sized cold heat bath to the hot heat bath. Hence, the temperature of the finite-sized cold heat bath decreases until it reaches the desired low-temperature $T_f$. By minimizing the input work for the heat transport process, we derive the optimal path for temperature change. We calculate the coefficient of performance as a function of average input power. We also obtain the bounds for the coefficient of performance by applying the asymmetric dissipation limits. For the parameter values considered in this study, we observe that the relation between the coefficient of performance and the average input power strongly depends on the nature of the finite-sized heat cold bath.

cond-mat.stat-mech↗

arXiv2026

Determining Critical Temperature Differences of Low-Temperature-Differential Stirling Engines: Nonlinear Dynamics Approach

While the low-temperature-differential (LTD) Stirling engines are innovative engines that can operate with low temperature differentials in our daily life, the problem of determining the critical temperature differences below which the engine ceases to rotate remains unexplored. In this study, we solve this problem using a nonlinear dynamics approach. We derive the self-consistent equations that determine the critical temperature differences as homoclinic bifurcation points of a dynamical model of the LTD Stirling engines. The solutions of the self-consistent equations reveal a combination of parameters that determines the critical temperature differences. This enables us to establish the fundamental design principles for improving the performance of the LTD Stirling engines beyond empirical designs.

nlin.AO↗

arXiv2026

Mean-Field Theory for Heider Balance under Heterogeneous Social Temperatures

Heider balance theory provides a fundamental framework for understanding the formation of friendly and hostile relations in social networks. Existing stochastic formulations typically assume a uniform social temperature, implying that all interpersonal relations fluctuate with the same intensity. However, studies show that social interactions are highly heterogeneous, with broad variability in stability, volatility, and susceptibility to change. In this work, we introduce a generalized Heider balance model on a complete graph in which each link is assigned its own social temperature. Within a mean-field formulation, we derive a distribution-dependent self-consistency condition for the collective opinion state and identify the criteria governing the transition between polarized and non-polarized configurations. This framework reveals how the entire distribution of interaction heterogeneity shapes the macroscopic behavior of the system. We show that the functional form of the inverse-temperature distribution, in particular whether it is light-tailed or heavy-tailed, leads to qualitatively distinct phase diagrams. We also establish universal bounds for the critical transition, where the homogeneous-temperature limit provides a universal lower bound for the critical mean of an inverse-temperature distribution governing the transition. Numerical simulations confirm the theoretical predictions and highlight the nontrivial effects introduced by heterogeneity. Our results provide a unified route to understanding structural balance in realistic social systems and lay the groundwork for extensions incorporating fluctuations beyond mean field, external fields, and network topologies beyond the complete graph.

physics.soc-ph↗

arXiv2025

Irreversibility of the pendulum revisited from Husimi's adiabaticity parameter

We revisit the irreversibility of the pendulum with time-dependent angular frequency, considered in a classical paper by K. Husimi. He introduced a parameter that measures the adiabaticity of a process utilizing an adiabatic invariant for the equation of motion of the pendulum. With this adiabaticity parameter, Husimi showed the irreversibility of the pendulum for a cyclic process, which is reminiscent of the Planck principle in thermodynamics, based on the microscopic mechanics. In this study, we generalize the argument by Husimi to a damped pendulum with friction, and highlight the role of conservation of a phase-space area on the Husimi's adiabaticity parameter. Moreover, we also investigate the second law of thermodynamics and its generalization for a general non-cyclic process as well as a cyclic process, and elucidate how the Husimi's adiabaticity parameter impacts on this law. In particular, for a general non-cyclic process, we show the law of entropy non-decrease for the pendulum without friction by using the property of the Husimi's adiabaticity parameter.

cond-mat.stat-mech↗

arXiv2025

Formulation of Entropy through Work by Carnot Machine and Direct Derivation of Law of Entropy Non-Decrease from Kelvin-Planck Principle

We derive the law of entropy non-decrease directly from the Kelvin-Planck principle for simple and compound systems without using the Clausius inequality. A key of the derivation is a new formulation of entropy in terms of work by a Carnot machine operating between a system and a single heat reservoir at fixed temperature, which is equivalent to Clausius entropy based on heat and Gyftopoulos-Beretta entropy based on work. We also show that we may characterize entropy as an extra thermodynamic cost that needs to be paid to create nonuniformity in the system.

cond-mat.stat-mech↗

arXiv2025

Nonequilibrium thermodynamics of populations of weakly-coupled low-temperature-differential Stirling engines with synchronous and asynchronous transitions

This study developed the theory of nonequilibrium thermodynamics for populations of low-temperature-differential (LTD) Stirling engines weakly-coupled in a general class of networks to clarify the effects of synchronous and asynchronous transitions on the power and thermal efficiency. We first show that synchronous (asynchronous) transitions increase (decrease) the power and thermal efficiency of weakly-coupled LTD Stirling engines based on quasilinear response relations between formally defined thermodynamic fluxes and forces. After that, we construct a conceptual model satisfying the quasilinear response relations to give a physical interpretation of the changes in power and thermal efficiency due to synchronous and asynchronous transitions, and justify the use of this conceptual model. We then show that the conceptual model, rather than the quasilinear response relations, preserves the thermodynamic irreversibility of the original model and thus gives more accurate results than those using the quasilinear response relations. Finally, we compare the dynamics between the original and the conceptual models for two-engine systems and show that the conceptual models roughly preserve the dynamical characteristics leading up to the synchronous transitions, while some detailed dynamical structures are lost.

cond-mat.stat-mech↗

arXiv2025

Decomposition of metric tensor in thermodynamic geometry in terms of relaxation timescales

Geometrical methods are extensively applied to thermodynamics including stochastic thermodynamics. In the case of slow-driving linear response regime, a geometrical framework, known as thermodynamic geometry, is established. The key of this framework is the thermodynamic length characterized by a metric tensor defined on the space of controlling variables. As the metric tensor is given in terms of the equilibrium time-correlation functions of the thermodynamic forces, it contains the information of timescales, which may be useful for analyzing the performance of heat engines. In this paper, we show that the metric tensor for underdamped Langevin dynamics can be decomposed in terms of the relaxation times of a system itself, which govern the timescales of the equilibrium time-correlation functions of the thermodynamic forces. As an application of the decomposition of the metric tensor, we demonstrate that it is possible to achieve Carnot efficiency at finite power by taking the vanishing limit of relaxation times without breaking trade-off relations between efficiency and power of heat engines in terms of thermodynamic geometry.

cond-mat.stat-mech↗

arXiv2024

Non-unique detailed constructions of Curzon-Ahlborn cycle on thermodynamic plane

The Curzon-Ahlborn (CA) cycle is a paradigmatic model of endoreversible heat engines, which yields the so-called CA efficiency as the efficiency at maximum power. Due to the arbitrariness of the relationship between the steady temperature and the time taken for the isothermal process of the CA cycle, the constructions of the CA cycle on the thermodynamic plane are not unique. Here, we give some of the detailed constructions of the CA cycle on the thermodynamic plane, using an ideal gas as a working substance. It is shown that these constructions are equal to each other in the maximum power regime in the sense that they achieve the best trade-off between the work and the inverse cycle-time, known as the Pareto front in multi-objective optimization problems.

cond-mat.stat-mech↗

arXiv2023

Synchronization approach to achieving maximum power and thermal efficiency for weakly-coupled low-temperature-differential Stirling engines

Low-temperature-differential (LTD) Stirling engines are heat engines that can operate autonomously with a slight temperature difference between low-temperature heat reservoirs and are thus expected to contribute to a sustainable society. A minimal dynamical-system model with only two variables has been proposed to explain the principle of autonomous rotational motion caused by temperature differences, and the maximum efficiency of the engine was formulated [Y. Izumida, Europhys. Lett. 121, 50004 (2018); Phys. Rev. E 102, 012142 (2020)]. This paper aims to clarify the coupling effects on the dynamics, power, and thermal efficiency of a pair of weakly coupled LTD Stirling engines and formulate the maximum thermal efficiency of the coupled system in the quasilinear response regime. We show that the dependence relation between the effective frequency difference and the coupling strength is characterized by a hysteresis, which comes from different kinds of bifurcations in the process of increasing and decreasing the value of the coupling strength. Then, by generalizing thermodynamic fluxes and forces and their quasilinear relations for engines under weak coupling, we show that the coupling improves the power exerted against the load torques and the thermal efficiency. We further show that their maximum values are achieved when the engines are synchronized. Since the thermal efficiency depends on the frequency difference, the dependence of thermal efficiency on the coupling strength is also characterized by a hysteresis. Finally, the load torque that achieves the maximum thermal efficiency of the coupled system is formulated.

nlin.AO↗

arXiv2023

Non-quasistatic response coefficients and dissipated availability for macroscopic thermodynamic systems

The characterization of finite-time thermodynamic processes is of crucial importance for extending equilibrium thermodynamics to nonequilibrium thermodynamics. The central issue is to quantify responses of thermodynamic variables and irreversible dissipation associated with non-quasistatic changes of thermodynamic forces applied to the system. In this study, we derive a simple formula that incorporates the non-quasistatic response coefficients with Onsager's kinetic coefficients, where the Onsager coefficients characterize the relaxation dynamics of fluctuation of extensive thermodynamic variables of semi-macroscopic systems. Moreover, the thermodynamic length and the dissipated availability that quantifies the efficiency of irreversible thermodynamic processes are formulated in terms of the derived non-quasistatic response coefficients. The present results are demonstrated by using an ideal gas model. The present results are, in principle, verifiable through experiments and are thus expected to provide a guiding principle for the nonequilibrium control of macroscopic thermodynamic systems.

cond-mat.stat-mech↗

arXiv2023

Thermodynamic efficiency of atmospheric motion governed by Lorenz system

The Lorenz system was derived on the basis of a model of convective atmospheric motions and may serve as a paradigmatic model for considering a complex climate system. In this study, we formulated the thermodynamic efficiency of convective atmospheric motions governed by the Lorenz system by treating it as a non-equilibrium thermodynamic system. Based on the fluid conservation equations under the Oberbeck-Boussinesq approximation,the work necessary to maintain atmospheric motion and heat fluxes at the boundaries were calculated. Using these calculations, the thermodynamic efficiency was formulated for stationary and chaotic dynamics. The numerical results show that, for both stationary and chaotic dynamics, the efficiency tends to increase as the atmospheric motion is driven out of thermodynamic equilibrium when the Rayleigh number increases. However, it is shown that the efficiency is upper bounded by the maximum efficiency, which is expressed in terms of the parameters characterizing the fluid and the convective system. The analysis of the entropy generation rate was also performed for elucidating the difference between the thermodynamic efficiency of conventional heat engines and the present atmospheric heat engine. It is also found that there exists an abrupt drop in efficiency at the critical Hopf bifurcation point, where the dynamics change from stationary to chaotic. These properties are similar to those found previously in Malkus-Lorenz waterwheel system.

nlin.CD↗

arXiv2023

Transportation efficiency of hydrodynamically coupled spherical oscillators in low Reynolds number fluids

Most bacteria are driven by the cilia or flagella, consisting of a long filament and a rotary molecular motor through a short flexible hook. The beating pattern of these filaments shows synchronization properties from hydrodynamic interactions, especially in low Reynolds number fluids. Here, we introduce a model based on simple spherical oscillators which execute oscillatory movements in one dimension by an active force, as a simplified imitation of the movements of cilia or flagella. It is demonstrated that the flow, measured by the net transportation of a test particle, is generated by a chain of oscillators and enhanced by the hydrodynamic interactions between beads, with supports from both perturbative and numerical results. Transportation efficiency also highly correlates with hydrodynamic interactions. Increments of bead numbers are generally expected to produce stronger flow and efficiency, at least for small numbers of beads.

physics.flu-dyn↗

arXiv2022

Irreversible efficiency and Carnot theorem for heat engines operating with multiple heat baths in linear response regime

The Carnot theorem, one expression of the second law of thermodynamics, places a fundamental upper bound on the efficiency of heat engines operating between two heat baths. The Carnot theorem can be stated in a more generalized form for heat engines operating with multiple heat baths, where the maximum efficiency is achieved for reversible heat engines operating quasistatically between two heat baths. In this study, we determine the irreversible efficiency of heat engines operating with multiple heat baths in a linear response regime, i.e., under small temperature differences and a slow variation of the control parameters, by quantifying the impact of the dissipation by irreversible operations. The Carnot theorem is derived as a natural consequence of it. Because the result obtained is based on the linear response relation and fluctuation-dissipation theorem in the universal framework of linear response theory, it has wide applicability to irreversible heat engines operating in the linear response regime.

cond-mat.stat-mech↗

arXiv2022

Achieving Carnot efficiency in a finite-power Brownian Carnot cycle with arbitrary temperature difference

Achieving the Carnot efficiency at finite power is a challenging problem in heat engines due to the trade-off relation between efficiency and power that holds for general heat engines. It is pointed out that the Carnot efficiency at finite power may be achievable in the vanishing limit of the relaxation times of a system without breaking the trade-off relation. However, any explicit model of heat engines that realizes this scenario for arbitrary temperature difference has not been proposed. Here, we investigate an underdamped Brownian Carnot cycle where the finite-time adiabatic processes connecting the isothermal processes are tactically adopted. We show that in the vanishing limit of the relaxation times in the above cycle, the compatibility of the Carnot efficiency and finite power is achievable for arbitrary temperature difference. This is theoretically explained based on the trade-off relation derived for our cycle, which is also confirmed by numerical simulations.

cond-mat.stat-mech↗

arXiv2021

Hierarchical Onsager symmetries in adiabatically driven linear irreversible heat engines

In existing linear response theories for adiabatically driven cyclic heat engines, Onsager symmetry is identified only phenomenologically, and a relation between global and local Onsager coefficients, defined over one cycle and at any instant of a cycle, respectively, is not derived. To address this limitation, we develop a linear response theory for the speed of adiabatically changing parameters and temperature differences in generic Gaussian heat engines obeying Fokker--Planck dynamics. We establish a hierarchical relationship between the global linear response relations, defined over one cycle of the heat engines, and the local ones, defined at any instant of the cycle. This yields a detailed expression for the global Onsager coefficients in terms of the local Onsager coefficients. Moreover, we derive an efficiency bound, which is tighter than the Carnot bound, for adiabatically driven linear irreversible heat engines based on the detailed global Onsager coefficients. Finally, we demonstrate the application of the theory using the simplest stochastic Brownian heat engine model.

cond-mat.stat-mech↗

arXiv2021

Compatibility of Carnot efficiency with finite power in an underdamped Brownian Carnot cycle in small temperature-difference regime

We study the possibility of achieving the Carnot efficiency in a finite-power underdamped Brownian Carnot cycle. Recently, it was reported that the Carnot efficiency is achievable in a general class of finite-power Carnot cycles in the vanishing limit of the relaxation times. Thus, it may be interesting to clarify how the efficiency and power depend on the relaxation times by using a specific model. By evaluating the heat-leakage effect intrinsic in the underdamped dynamics with the instantaneous adiabatic processes, we demonstrate that the compatibility of the Carnot efficiency and finite power is achieved in the vanishing limit of the relaxation times in the small temperature-difference regime. Furthermore, we show that this result is consistent with a trade-off relation between power and efficiency by explicitly deriving the relation of our cycle in terms of the relaxation times.

cond-mat.stat-mech↗