Frequency dependence of giant Shapiro steps in ordered and site-disordered proximity-coupled Josephson-junction arrays

Abstract

We present experimental results on the frequency dependence of the integer giant Shapiro steps on ordered and random site-disordered arrays of Nb-Au-Nb Josephson junctions. We find that the dependence of the Shapiro step heights upon the rf frequency follows the predictions of Octavio et al. for the resistively shunted junction model. We also find that increasing the disorder greatly suppresses the step heights and that at high frequencies the maximum step heights exhibit an extremely pronounced rolloff, in direct contrast to arrays with no disorder. The Shapiro steps for the site-disordered samples show significant voltage broadening which increases linearly with frequency. This behavior appears to be a unique feature in the site-disorder systems, which we qualitatively interpret as arising from current-induced vortices generated at the defect sites.