Entropy production, energy loss and currents in adiabatically rocked thermal ratchets

We study the nature of currents, input energy and entropy production in different types of adiabatically rocked ratchets using the method of stochastic energetics. The currents exhibit a peak as a function of noise strength. We show that there is no underlying resonance or synchronisation phenomena in the dynamics of the particle with these current peaks. This follows from the analysis of energy loss in the medium. We also show that the maxima seen in current as well as the total entropy production are not directly correlated.Comment: 6 pages, 7 figures Minor corrections are mad

Transport Coherence in Frictional Ratchets

We study the phenomena of noise induced transport in frictional ratchet systems. For this we consider a Brownian particle moving in a space dependent frictional medium in the presence of external white noise fluctuations. To get the directed transport, unlike in other ratchet models like flashing or rocking ratchets etc., we do not require the potential experienced by the particle to be asymmetric nor do we require the external fluctuations to be correlated. We have obtained analytical expressions for current and the diffusion coefficient. We show that the frictional ratchet do not exhibit a pronounced coherence in the transport in that the diffusion spread overshadows the accompanying directed transport in system with finite spatial extensions.Comment: Based on the poster presentation (by RK) at the Condensed Matter Days - 2003 held at Jadavpur University, Kolkata, India from 27-29 August 2003. Minor corrections have been don

Enhanced thermodynamic efficiency in time asymmetric ratchets

The energetic efficiency of an overdamped Brownian particle in a sawtooth potential in the presence of time asymmetric forcing is studied in the adiabatic limit. An error made in earlier work on the same problem in the literature is corrected. We find that asymmetry in the potential together with temporal asymmetry in the forcing leads to much enhanced efficiency without fine-tuning of the parameters. The origin of this is traced to the suppression of the backward current. We also present a comparative study of the roles of continuous and discontinuous ratchet forces as regards these measurable quantities. We find that the thermal fluctuations can optimize the energy transduction, the range of parameters, however, being very small. This ratchet model also displays current reversals on tuning of parameters even in the adiabatic regime. The possible relationships between the nature of the currents, entropy production and input energy are also addressed

Self-tuning of threshold for a two-state system

A two-state system (TSS) under time-periodic perturbations (to be regarded as input signals) is studied in connection with self-tuning (ST) of threshold and stochastic resonance (SR). By ST, we observe the improvement of signal-to-noise ratio (SNR) in a weak noise region. Analytic approach to a tuning equation reveals that SNR improvement is possible also for a large noise region and this is demonstrated by Monte Carlo simulations of hopping processes in a TSS. ST and SR are discussed from a little more physical point of energy transfer (dissipation) rate, which behaves in a similar way as SNR. Finally ST is considered briefly for a double-well potential system (DWPS), which is closely related to the TSS

Brownian motors: Joint effect of non-Gaussian noise and time asymmetric forcing

Previous works have shown that time asymmetric forcing on the one hand, as well as non-Gaussian noises on the other, can separately enhance the efficiency and current of a Brownian motor. Here, we study the result of subjecting a Brownian motor to both effects simultaneously. Our results have been compared with those obtained for the Gaussian white noise regime in the adiabatic limit. We find that, although the inclusion of the time asymmetry parameter increases the efficiency value up to a certain extent, for the present case this increase is much less appreciable than in the white noise case. We also present a comparative study of the transport coherence in the context of colored noise. Though the efficiency in some cases becomes higher for the non-Gaussian case, the P\'eclet number is always higher in the Gaussian colored noise case than in the white noise as well as non-Gaussian colored noise cases.Comment: 18 page

Noise induced currents and reliability of transport in frictional ratchets

We study the coherence of transport of an overdamped Brownian particle in frictional ratchet system in the presence of external Gaussian white noise fluctuations. The analytical expressions for the particle velocity and diffusion coefficient are derived for this system and the reliability or coherence of transport is analysed by means of their ratio in terms of a dimensionless P$\acute{e}$clet number. We show that the coherence in the transport can be enhanced or degraded depending sensitively on the frictional profile with respect to the underlying potential.Comment: 7 pages, 6 figure

Brownian rectifiers in the presence of temporally asymmetric unbiased forces

The efficiency of energy transduction in a temporally asymmetric rocked ratchet is studied. Time asymmetry favours current in one direction and suppresses it in the opposite direction due to which large efficiency ~ 50% is readily obtained. The spatial asymmetry in the potential together with system inhomogeneity may help in further enhancing the efficiency. Fine tuning of system parameters considered leads to multiple current reversals even in the adiabatic regime

Hartman effect in presence of Aharanov Bohm flux

The Hartman effect for the tunneling particle implies the independence of group delay time on the opaque barrier width, with superluminal velocities as a consequence. This effect is further examined on a quantum ring geometry in the presence of Aharonov-Bohm flux. We show that while tunneling through an opaque barrier the group delay time for given incident energy becomes independent of the barrier thickness as well as the magnitude of the flux. The Hartman effect is thereby extended beyond one dimension and in the presence of Aharonov-Bohm flux.Comment: 4 pages, 4 figure