GIANT SHAPIRO STEPS WITH SCREENING CURRENTS

We present a detailed study of the dynamic response of overdamped Josephson-junction square arrays driven by a dc plus ac current, including self-induced magnetic-field effects, as a function of a parameter-kappa. The transition from type-II (kappa > 1 ) to type-I (kappa < 1 ) behavior is studied in detail. It is found that the IV characteristics show fractional giant Shapiro steps, with and without external magnetic fields, with microscopic coherent vortex dynamics fundamentally different in the two extreme kappa-limits. Our results in zero external magnetic field are in good agreement with recent experiments in Nb-Au-Nb arrays.