Moving vortex matter with coexisting vortices and anti-vortices

Moving vortex matter, driven by transport currents independent of time, in which vortices and anti-vortices coexist is investigated theoretically in thin superconducting films with nanostructured defects. A simple London model is proposed for the vortex dynamics in films with periodic arrays of nanomagnets or cylindrical holes (antidots). Common to these films is that vortex anti-vortex pairs may be created in the vicinity of the defects by relatively small transport currents, because it adds to the current generated by the defects - the nanomagnets screening current, or the antidots backflow current - and may exceed locally the critical value for vortex anti-vortex pair creation. The model assumes that vortex matter dynamics is governed by Langevin equations, modified to account for creation and annihilation of vortex anti-vortex pairs. For pair creation, it is assumed that whenever the total current at some location exceeds a critical value, equal to that needed to separate a vortex from an anti-vortex by a vortex core diameter, a pair is created instantaneously around this location. Pair annihilation occurs by vortex anti-vortex collisions. The model is applied to films at zero external magnetic field and low temperatures. It is found that several moving vortex matter steady-states with equal numbers of vortices and anti-vortices are possible.