No abstract

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 tau and noise strength Q) the nonmonotonic variation of the mean escape time T as a function of tau 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 tau of the order of the relaxation time toward the metastable state.

An approximate method for studying activation over a fluctuating barrier of potential is proposed. It involves considering separately the slow and fast components of barrier fluctuations, and it applies for any value of their correlation time τ . It gives exact results for the limiting values τ → 0 and τ → ∞, and the agreement with numerics in between is also excellent, both for dichotomic and Gaussian barrier perturbations. Ever since Kramers seminal paper [1] the fluctuational escape over a potential barrier has been a paradigm for a thermal activation process. Recently, activation in the presence of time-varying fields have become a subject of great interest due to the discovery of many counterintuitive noise-assisted effects, like stochastic resonance [2] or transport in Brownian motors [3]. The nonequilibrium character of these problems hinders, however, the direct application of many ideas and methods developed for investigation of the static Kramers problem [4] (e.g., detailed balance or rate concept). On the other hand, as the time-scale of variation of the driving signal is independent of the internal dynamics of the system, standard adiabatic methods are restricted to certain ranges of parameters, only. Hence, an approach which overcomes these difficulties and applies for the whole range of time variability of the perturbation, is of great importance.

We investigate the problem of tunneling across a randomly fluctuating barrier in the presence of dissipation in the two-level approximation. The barrier fluctuations are induced by a random telegraph noise whose switching rate nu is taken as a control parameter. For infinitely fast fluctuations the dynamics of the system is similar to the static case, while, for very small nu, the barrier evolution is a superposition of static solutions for both configurations. This leads to a resonant beating or long-time periodic localization. For an intermediate value of nu we have found a resonancelike suppression of coherent tunneling. When the system levels are detuned, a resonant enhancement of decay in the incoherent regime also occurs.