Jing-Dong Bao

Systematic studies of fission fragment kinetic energy distributions by Langevin simulations

Fission fluctuation-dissipation dynamics of heavy nuclei has been studied using Langevin Monte Carlo simulations. The covariant form of the fission transport equation and the coefficients related to it are investigated. To learn about the influence of the dynamics from the ground state to the saddle point on the kinetic energy distributions we have studied various systems and compared the calculations both starting from the ground state and from the saddle point. Both the mean total kinetic energy of the fission fragments and its variances can fit with the experimental values in terms of a finite neck radius as scission condition.

Intermediate dynamics between Newton and Langevin.

A dynamics between Newton and Langevin formalisms is elucidated within the framework of the generalized Langevin equation. For thermal noise yielding a vanishing zero-frequency friction the corresponding non-Markovian Brownian dynamics exhibits anomalous behavior which is characterized by ballistic diffusion and accelerated transport. We also investigate the role of a possible initial correlation between the system degrees of freedom and the heat-bath degrees of freedom for the asymptotic long-time behavior of the system dynamics. As two test beds we investigate (i) the anomalous energy relaxation of free non-Markovian Brownian motion that is driven by a harmonic velocity noise and (ii) the phenomenon of a net directed acceleration in noise-induced transport of an inertial rocking Brownian motor.

Time-dependent barrier passage of a non-Ohmic damping system.

We consider a particle passing over the saddle point of an inverse harmonic potential, which is described by a generalized Langevin equation with a non-Ohmic damping of power exponent delta. The time-dependent passing probability and transmission coefficient are obtained analytically by using the reaction flux method. It is shown that the overshooting phenomenon for the passing probability appears in the regime 0<delta<1 and the backflow recrossing over the saddle point is observed, where a nonmonotonous time dependence of the passage probability is detected. The long memory aspect of friction is at the origin of this behavior. Thus the steady transmission coefficient is also a nonmonotonous function of delta.

Directed current of Brownian ratchet randomly circulating between two thermal sources

We consider a Brownian ratchet system which circulates fast and randomly between two thermal sources with different temperatures. The steady current of the system is evaluated by the Langevin simulations. An analytical expression of the current for fast exchange of the noises is obtained. The feature of temperature-dependent current is discussed. The results show that the current for randomly exchanging two white noises is larger than that of periodically changing temperature under the same parameters.

EFFECTS OF MULTIPLICATIVE NOISE ON FLUCTUATION-INDUCED TRANSPORT

Abstract We study the transport of a Brownian particle moving in a ratchet-like periodic potential driven by multiplicative colored noise, and an analytic expression for the steady current with weak noise-strength is obtained. The results show that the effect of the multiplicative noise is equivalent to additive noise acting on a particle moving in an effective potential with a slope modified from the original one. Then a net steady current may arise even in the cases of white noise and a symmetric periodic potential. Some simple models are studied and discussed.

Efficiency of energy transformation in an underdamped diffusion ratchet

Abstract The transport of an underdamped Brownian particle moving in a one-dimensional ratchet driven by a modulated Gaussian white noise is considered. The system experiences periodically higher temperature T 1 during a time τ 1 and lower temperature T 2 during a time τ 2 . Both the average current and the thermal efficiency are calculated numerically. The results show that the efficiency of energy transformation can be optimized for a ratio of τ 1 / τ 2 less than 1 at a critical damping.

Inhomogeneous friction leading to current in periodic system

We study the diffusion process of a Brownian particle moving in a one-dimensional ratchet with space-dependent friction that is subjected to an external source. The equation of motion for a particle involves a multiplicative fluctuation, a nonlinear friction and an external driving force or a Gaussian white noise. The average position of the particle is simulated numerically in terms of the Langevin Monte Carlo method and discussed by means of the adiabatic approximation and the effective potential. The influence of coordinate-dependent friction on the average position and the direction of current of the particle is investigated. The results show that a net drift can be produced in the presence of both a coordinate-dependent friction and an external fluctuation, even when the ratchet potential and the temporal force are completely symmetrical.

Brownian free energy ratchets

The directed motion of a Brownian particle subject to a unequilibrium fluctuation which moves in a two-dimensional ratchet potential is studied. Thus, a working model of the free-energy ratchet is proposed through eliminating no-transport variables. The properties of the present model such as the current, the efficiency, the diffusion coefficient and the Pe number are discussed. Comparison with the previous one-dimensional models, some novel phenomena are observed.

Kramers escape rate in nonlinear diffusive media.

In this paper, we study nonlinear Kramers problem by investigating overdamped systems ruled by the one-dimensional nonlinear Fokker-Planck equation. We obtain an analytic expression for the Kramers escape rate under quasistationary conditions by employing a metastable potential and its predictions are in excellent agreement with numerical simulations. The results exhibit the anomalies due to the nonlinearity in W that the escape rate grows with D and drops as mu becomes large at a fixed D. Indeed, particles in the subdiffusive media (mu>1) can escape over the barrier only when D is above a critical value, while this confinement does not exist in the superdiffusive media (mu<1).

Stochastic resonance in multidimensional periodic potential

We study the mobility and diffusion of an underdamped Brownian particle moving in a two-dimensional (2D) periodic potential which subjects to a thermal white noise and a weak external driving force. Both the signal power amplification and the diffusion rate are calculated via Langevin simulations. It is shown that the stochastic resonance (SR) can be observed in the two dimension, namely, the output quantities as functions of the temperature show a nonmonotonic behavior, however, the SR cannot be obtained in the one dimension (1D). In the 2D potential, the height of dynamical barrier is decreased effectively along the direction of transport if the curvature of the potential at the barrier is less than that at the local minima. This leads to the SR condition being obeyed, i.e., the Kramers frequency over the barrier roughly matches the frequency of external signal.