23 research outputs found
Influence of the Barrier Shape on Resonant Activation
The escape of a Brownian particle over a dichotomously fluctuating barrier is
investigated for various shapes of the barrier. The problem of resonant
activation is revisited with the attention on the effect of the barrier shape
on optimal value of the mean escape time in the system. The characteristic
features of resonant behavior are analyzed for barriers switching either
between different heights, or "on" and "off" positions. PACS number(s):
05.10-a, 02.50.-r, 82.20.-wj.Comment: 7 pages, 8 figures, RevTex4. Manuscript has been revised and
enhanced. Pictures have been made more clear and some of them have been
cancelled. Additional references have been added. The paper has been
submitted to Phys. Rev.
Stochastic Resonance in a Dipole
We show that the dipole, a system usually proposed to model relaxation
phenomena, exhibits a maximum in the signal-to-noise ratio at a non-zero noise
level, thus indicating the appearance of stochastic resonance. The phenomenon
occurs in two different situations, i.e. when the minimum of the potential of
the dipole remains fixed in time and when it switches periodically between two
equilibrium points. We have also found that the signal-to-noise ratio has a
maximum for a certain value of the amplitude of the oscillating field.Comment: 4 pages, RevTex, 6 PostScript figures available upon request; to
appear in Phys. Rev.
Quantum dynamics in strong fluctuating fields
A large number of multifaceted quantum transport processes in molecular
systems and physical nanosystems can be treated in terms of quantum relaxation
processes which couple to one or several fluctuating environments. A thermal
equilibrium environment can conveniently be modelled by a thermal bath of
harmonic oscillators. An archetype situation provides a two-state dissipative
quantum dynamics, commonly known under the label of a spin-boson dynamics. An
interesting and nontrivial physical situation emerges, however, when the
quantum dynamics evolves far away from thermal equilibrium. This occurs, for
example, when a charge transferring medium possesses nonequilibrium degrees of
freedom, or when a strong time-dependent control field is applied externally.
Accordingly, certain parameters of underlying quantum subsystem acquire
stochastic character. Herein, we review the general theoretical framework which
is based on the method of projector operators, yielding the quantum master
equations for systems that are exposed to strong external fields. This allows
one to investigate on a common basis the influence of nonequilibrium
fluctuations and periodic electrical fields on quantum transport processes.
Most importantly, such strong fluctuating fields induce a whole variety of
nonlinear and nonequilibrium phenomena. A characteristic feature of such
dynamics is the absence of thermal (quantum) detailed balance.Comment: review article, Advances in Physics (2005), in pres
Resonant activation: Potential vs. temperature fluctuations
The thermal activation over a potential barrier in a stochastically modified system is analyzed. If colored noise perturbs the potential one can observe the famous resonant activation phenomenon. We compare this effect with the results caused by correlated fluctuations of the temperature. For high potential barrier stochastic variations of temperature lead to resonant activation as well. However, for low-barrier or barrierless evolution this effect disappears. We formulate an analytical condition separating these cases and present a physical interpretation of both types of behavior
Resonances while surmounting a fluctuating barrier.
Electronic analog experiments on escape over a fluctuating potential barrier are performed for the case when the fluctuations are caused by Ornstein-Uhlenbeck noise (OUN). In its dependence on the relation between the two OUN parameters (the correlation time t and noise strength Q) the nonmonotonic variation of the mean escape time T as a function of t can exhibit either a minimum (resonant activation), or a maximum (inhibition of activation), or both these effects. The possible resonant nature of these features is discussed. We claim that T is not a good quantity to describe the resonancelike character of the problem. Independently of the specific relation between the OUN parameters, the resonance manifests itself as a maximal lowering of the potential barrier during the escape event, and it appears for t of the order of the relaxation time toward the metastable stat
Transient multimodality in relaxation from an unstable state.
We analyse a relaxation process from an unstable state during which transient multimodality occurs. This phenomenon is investigated experimentally on an electronic analogue circuit which mimics an overdamped bistable oscillator described by a sixth--order polynomial U(x) and is driven by a Gaussian white noise. The measured times and positions at which new maxima appear in the probability distribution function agree well with the theoretical predictions. It is shown that the shape of the potential guarantees the existence of two different time scales, allowing for the coexistence of three probability distribution peaks during a sizeable interval of time, even though there is no long "flat" region in the potential where U'(x) is very small. Finally, the concept of marginality with reference to unsteady states is discussed
Properties of resonant activation phenomena
The phenomenon of resonant activation of a Brownian particle over a fluctuating barrier is revisited. We discuss the important distinctions between barriers that can fluctuate among up and down configurations, and barriers that are always up but that can fluctuate among different heights. A resonance as a function of the barrier fluctuation rate is found in both cases, but the nature and physical description of these resonances is quite distinct. The nature of the resonances, the physical basis for the resonant behavior, and the importance of boundary conditions are discussed in some detail. We obtain analytic expressions for the escape time over the barrier that explicitly capture the minima as a function of the barrier fluctuation rate, and show that our analytic results are in excellent agreement with numerical results