Metastability and Uniqueness of Vortex States at Depinning

We present results from numerical simulations of the transport of vortices in the zero-field-cooled (ZFC) and the field-cooled (FC) state of a type-II superconductor. In the absence of an applied current I, we find that the FC state has a lower defect density than the ZFC state, and is stable against thermal cycling. On the other hand, by cycling I, surprisingly, we find that the ZFC state is the stable state. The FC state is metastable as manifested by increasing I to the depinning current I_c, in which case the FC state evolves into the ZFC state. We also find that all configurations acquire a unique defect density at the depinning transition independent of the history of the initial states.