FREQUENCY-DEPENDENCE OF GIANT SHAPIRO STEPS IN ORDERED AND SITE-DISORDERED PROXIMITY-COUPLED JOSEPHSON-JUNCTION ARRAYS

We present experimental results on the frequency dependence of the int eger giant Shapiro steps on ordered and random site-disordered arrays of Nb-Au-Nb Josephson junctions. We find that the dependence of the Sh apiro step heights upon the rf frequency follows the predictions of Oc tavio et al. for the resistively shunted junction model. We also find that increasing the disorder greatly suppresses the step heights and t hat 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 voltag e broadening which increases linearly with frequency. This behavior ap pears to be a unique feature in the site-disorder systems, which we qu alitatively interpret as arising from current-induced vortices generat ed at the defect sites.