Tatsuro Yuge

Nonequilibrium Molecular Dynamics Simulation of Electric Conduction

We propose a realistic model for electric conduction, and study transport phenomena by molecular dynamics simulation. We observe that the system reaches a nonequilibrium steady state in the presence of an external electric field. The electrical conductivity is almost independent of the impurity distribution and the system size, and there is no long-time tail. The fluctuation–dissipation theorem and the Kramers–Kronig relation hold at all frequencies. These results show that this model has normal transport properties.

General Properties of Response Functions of Nonequilibrium Steady States

We derive general properties, which hold for both quantum and classical systems, of response functions of nonequilibrium steady states. We clarify differences from those of equilibrium states. In particular, sum rules and asymptotic behaviors are derived, and their implications are discussed. Since almost no assumptions are made, our results are applicable to diverse physical systems. We also demonstrate our results by a molecular dynamics simulation of a many-body interacting system.We derive general properties, which hold for both quantum and classical systems, of response functions of nonequilibrium steady states. We clarify differences from those of equilibrium states. In particular, sum rules and asymptotic behaviors are derived, and their implications are discussed. Since almost no assumptions are made, our results are applicable to diverse physical systems. We also demonstrate our results by a molecular dynamics simulation of a many-body interacting system.

Sum rule for response function in nonequilibrium Langevin systems.

We derive general properties of the linear-response functions of nonequilibrium steady states in Langevin systems. These correspond to extension of the results which were recently found in Hamiltonian systems [A. Shimizu and T. Yuge, J. Phys. Soc. Jpn. 79, 013002 (2010)]. We discuss one of the properties, the sum rule for the response function, in particular detail. We show that the sum rule for the response function of the velocity holds in the underdamped case, whereas it is violated in the overdamped case. This implies that the overdamped Langevin models should be used with great care. We also investigate the relation of the sum rule to an equality on the energy dissipation in nonequilibrium Langevin systems, which was derived by Harada and Sasa.

Indications of Universal Excess Fluctuations in Nonequilibrium Systems

The fluctuation in electric current in nonequilibrium steady states is investigated by molecular dynamics simulation of macroscopically uniform conductors. At low frequencies, appropriate decomposition of the spectral intensity of current into thermal and excess fluctuations provides a simple picture of excess fluctuations behaving as shot noise. This indicates that the fluctuation– dissipation relation may be violated in a universal manner by the appearance of shot noise for a wide range of systems with particle or momentum transport.

Markovian Quantum Master Equation beyond Adiabatic Regime

By introducing a temporal change time scale τ_{A}(t) for the time-dependent system Hamiltonian, a general formulation of the Markovian quantum master equation is given to go well beyond the adiabatic regime. In appropriate situations, the framework is well justified even if τ_{A}(t) is faster than the decay time scale of the bath correlation function. An application to the dissipative Landau-Zener model demonstrates this general result. The findings are applicable to a wide range of fields, providing a basis for quantum control beyond the adiabatic regime.

Long-time behavior of velocity autocorrelation function for interacting particles in a two-dimensional disordered system

The long-time behavior of the velocity autocorrelation function (VACF) is investigated by the molecular dynamics simulation of a two-dimensional system which has both a many-body interaction and a random potential. With strengthening the random potential by increasing the density of impurities, a crossover behavior of the VACF is observed from a positive tail, which is proportional to t -1 , to a negative tail, proportional to - t -2 . The latter tail exists even when the density of particles is the same order as the density of impurities. The behavior of the VACF in a nonequilibrium steady state is also studied. In the linear response regime the behavior is similar to that in the equilibrium state, whereas it changes drastically in the nonlinear response regime.

Long-Time Tail in an Electric Conduction System

The long-time behavior of the velocity autocorrelation function in a classical twodimensional electric conduction system is studied by the molecular dynamics simulation. In equilibrium, the effect of coexistence of many-body interactions and a random potential is investigated. A crossover from a positive tail proportional to t −1 , to a negative tail proportional to −t −2 is observed as the strength of the random potential increases. In nonequilibrium, the positive tail is enhanced whereas the negative tail appears at earlier times as an electric fi eld increases.

A Perturbative Method for Nonequilibrium Steady State of Open Quantum Systems

We develop a method of calculating the nonequilibrium steady state (NESS) of an open quantum system that is weakly coupled to reservoirs in different equilibrium states. We describe the system using a Redfield-type quantum master equation (QME). We decompose the Redfield QME into a Lindblad-type QME and the remaining part

Sum Rules and Asymptotic Behaviors for Optical Conductivity of Nonequilibrium Many-Electron Systems

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Cavity-Loss Induced Plateau in Coupled Cavity QED Array

. Regarding the steady state of the Lindblad QME as the unperturbed solution, we perform a perturbative calculation with respect to