Shapiro steps in driven vortex lattices interacting with periodic pinning arrays

We examine the effects of DC and AC driving on vortex lattices in periodic pinning arrays for a system containing twice as many vortices as pinning si tes. The vortex flow occurs by the motion of the interstitial vortices thro ugh a periodic potential generated by the vortices that remain pinned at th e pinning sites. With an AC offset, we observe Shapiro steps when the inter stitial vortex motion interferes with the applied AC driving. The widths of the Shapiro steps as a function of the magnitude of the AC driving are fou nd to follow a Bessel function in agreement with theory. (C) 2000 Elsevier Science B.V. All rights reserved.