Depinning of Two-Dimensional Vortex System with Square Pinning Array at the Second Matching Field

Depinning of a two-dimensional vortex system at the second matching field B=2B _? in the presence of square pinning array was studied by molecular dynamics simulations. The annealed ground structures and depinning properties are found to be dependent on pinning strength. For strong pinning, we find a two-step depinning transition with a lower depinning force F _c1 and a higher one F _c2. At F _c1 only the interstitial vortices start to move, while at F _c2 the vortices at pinning sites are depinned and all vortices move.