Non linear dynamics of vortices in superconductors with short coherence length

In superconductors where the coherence length is comparable to the Fermi wavelength, the vortex viscosity depends on the velocity of the vortex, leading to non linear equations of motion. The trajectories of vortices driven by a. c. fields show a variety of behaviors as function of frequency. Finite pertubations give rise to very long lived transients. The relevance of these results to experiments in high-T$_{\rm c}$ superconductors is discussed.