Shapiro steps in a superconducting film with an antidot lattice

Shapiro voltage steps at voltages V-n = nV(0) (n integer) have been observe d in the voltage-current characteristics of a superconducting film with a s quare lattice of perforating microholes (antidots) in the presence of radio frequent radiation. These equidistant steps appear at the second matching f ield H-2 when the flow of the interstitial vortex lattice in the periodic p otential created by the antidots and the vortices trapped by them, is in ph ase with the applied rf frequency. Therefore, the observation of Shapiro st eps clearly reveals the presence of mobile interstitial vortices in superco nducting films with regular pinning arrays. The interstitial vortices, move d by the driving current, coexist with immobile vortices strongly pinned at the antidots. [S0163-1829(99)51434-3].