Hui-Zhang Xie

Heat flow and efficiency in a microscopic engine

Abstract.We study the energetics of a thermal motor driven by temperature differences, which consists of a Brownian particle moving in a sawtooth potential with an external load where the viscous medium is periodically in contact with hot and cold heat reservoir along space coordinate. The motor can work as a heat engine or a refrigerator under different conditions. The heat flow via both potential and kinetic energy is considered. The former is reversible when the engine works quasistatically and the latter is always irreversible. The efficiency of the heat engine can never approach Carnot efficiency.

Thermal noise can facilitate energy transformation in the presence of entropic barriers.

Efficiency of a Brownian particle moving along the axis of a three-dimensional asymmetric periodic channel is investigated in the presence of a symmetric unbiased force and a load. Reduction of the spatial dimensionality from two or three physical dimensions to an effective one-dimensional system entails the appearance of entropic barriers and an effective diffusion coefficient. The energetics in the presence of entropic barriers exhibits peculiar behavior which is different from that occurring through energy barriers. We found that even on the quasistatic limit there is a regime where the efficiency can be a peaked function of temperature, which indicates that thermal noise can facilitate energy transformation, contrary to the case of energy barriers. The appearance of entropic barriers may induce optimized efficiency at a finite temperature.

Efficiency optimization in a correlation ratchet with asymmetric unbiased fluctuations

The efficiency of a Brownian particle moving in a periodic potential in the presence of asymmetric unbiased fluctuations is investigated. We found that even on the quasistatic limit there is a regime where the efficiency can be a peaked function of temperature, which proves that thermal fluctuations facilitate the efficiency of energy transformation, contradicting the earlier findings [H. Kamegawa et al., Phys. Rev. Lett. 80, 5251 (1998)]. It is also found that the mutual interplay between temporal asymmetry and spatial asymmetry may induce optimized efficiency at finite temperatures. The ratchet is not most efficient when it gives maximum current.

Efficiency and current in a correlated ratchet

We present a detailed study of the transport and the efficiency of a ratchet system in a periodic potential in the presence of correlated noises. The current and the efficiency of the system are investigated. It is found that, when the potential is spatially symmetric, the correlation between the two noises can induce a net transport. The efficiency shows many interesting features as a function of the applied force, the noise intensity, the external load, etc. The efficiency can be maximized as a function of noise intensity (or temperature), which shows that the thermal fluctuation can facilitate the efficiency of energy transformation.

Current reversals in an inhomogeneous system with asymmetric unbiased fluctuations

Abstract.We present a study of the transport of a Brownian particle moving in a periodic symmetric potential in the presence of asymmetric unbiased fluctuations. The particle is considered to move in a medium with periodic space dependent friction. By tuning the parameters of the system, the direction of the current exhibits reversals, both as a function of temperature as well as the amplitude of rocking force. We found that the mutual interplay between the opposite driving factors is the necessary term for current reversals.

Efficiency in a temporally asymmetric Brownian motor with stochastic potentials

The efficiency of a Brownian motor moving in a periodic potential is investigated in the presence of a position-dependent stochastic perturbation and a temporally asymmetric unbiased external force. The competition between two or more driving factors may induce optimized efficiency at finite temperatures, which indicates that thermal fluctuations facilitate the efficiency of energy transformation. There is an optimized value of the asymmetric parameter of external force at which the efficiency exhibits its maximum. It is also found that the potential fluctuations can facilitate energy conversion.

Transient properties of a bistable kinetic model with quantum corrections

The transient properties of a Brownian particle moving in a bistable system with quantum corrections are investigated. The Quantum Smoluchowski Equation (QSE) is fully valid for high temperatures; for low temperatures it is valid only in a restricted domain of the state space. The quantum effects in a bistable system stand out for low temperatures. Explicit expressions of the mean first-passage time (MFPT) are obtained by using a steepest-descent approximation. The quantum effects are against the particle moving towards the destination from its original position.

BISTABLE KINETIC MODEL DRIVEN BY AN INHOMOGENEOUS NOISE

Steady state properties of a Brownian particle moving in a bistable kinetic system is investigated in the presence of an inhomogeneous noise. It is found that the inhomogeneous noise makes the system change from two-state system to one-state system, namely, the inhomogeneous noise can induce a phase transition. The particle will always move from a large intensity noise region to a small intensity region.