Scaling Behaviors and Novel Creep Motion of Flux Lines under AC Driving

We performed Langevin dynamics simulations for the \textit{ac} driven flux lines in a type II superconductor with random point-like pinning centers. Scaling properties of flux-line velocity with respect to instantaneous driving force of small frequency and around the critical \textit{dc} depinning force are revealed successfully, which provides precise estimates on dynamic critical exponents. From the scaling function we derive a creep law associated with the activation by the regular shaking. The effective energy barrier vanishes at the critical dc depinning point in a square-root way when the instantaneous driving force increases. The frequency plays a similar role of temperature in conventional creep motions, but in a nontrivial way governed by the critical exponents. We have also performed systematic finite-size scaling analysis for flux-line velocity in transient processes with \textit{dc} driving, which provide estimates on critical exponents in good agreement with those derived with ac driving. The scaling law is checked successfully.