Phase-locking of vortex lattices interacting with periodic pinning

We examine Shapiro steps for vortex lattices interacting with periodic pinn ing arrays driven by ac and de currents. The vortex flow occurs by the moti on of the interstitial vortices through the periodic potential generated by the vortices that remain pinned at the pinning sites. Shapiro steps are ob served for fields B-phi<B<2.25B(phi) with the most pronounced steps occurri ng for fields where the interstitial vortex lattice has a high degree of sy mmetry. The widths of the phase-locked current steps as a function of the m agnitude of the ac driving are found to follow a Bessel function in agreeme nt with theory.