Thermally Activated Flux Creep in High-Temperature Superconductors: a Stochastic Model

A model is presented which describes magnetic properties of high-Tc superconductors when flux creep is the leading mechanism for dissipation. This model reduces the behaviour of a complex current pattern to the behaviour of essentially one single current loop which contains a fluctuating number of flux quanta. It is shown that the dynamics of such a system describes widely observed phenomena such as logarithmic decay of the magnetization, broadening of the resistive transition as a function of an external field and nonlinearity in the a.c.-susceptibility. Away from equilibrium, fluctuations in the magnetization and even harmonics in the a.c.-susceptibility decay algebraically in time.