RESONANT STEPS IN PARALLEL JOSEPHSON-JUNCTION ARRAYS - PARAMETRIC-INSTABILITIES OF WHIRLING MODES

Circular arrays of underdamped Josephson junctions exhibit a series of resonant steps in the return path of the subgap region in the current -voltage characteristics. We show that the voltage locations of the st eps can be predicted by studying the parametric instabilities of whirl ing periodic solutions, and experimentally verify the prediction in a ring of 8 underdamped junctions. The whirling modes become unstable in certain voltage intervals, and a branch (a resonant step) of more com plicated solutions emerges from the endpoint of each interval. We exte nd the analysis to open-ended arrays and find that for f = 0, the onse t of a zero-field step has the same underlying mechanism. For f > 0, c ombinations of lattice eigenfrequencies are excited.