  A detailed numerical study on the directed motion of ac-driven vortices and
antivortices in 2D Josephson junction arrays (JJA) with an asymmetric periodic
pinning potential is reported. Dc-voltage rectification shows a strong
dependence on vortex density as well as an inversion of the vortex flow
direction with ac amplitude for a wide range of vortex density around $f$=1/2
($f$=$Ha^2 / \Phi_0$), in good agreement with recent experiments by Shal\'om
and Pastoriza [Phys. Rev. Lett. {\bf 94}, 177001 (2005)]. The study of vortex
structures, spatial and temporal correlations, and vortex-antivortex pairs
formation gives insight into a purely collective mechanism behind the current
reversal effect.

Superconductivity