Modulation of low—frequency shapiro steps of superconducting proximity-effect junctions

Abstract

We observe a modulation of Shapiro step heights in superconducting Nb-Au-Nb (SNS) Josephson junctions at an rf frequency v = 20 MHz (hv/2eRI c ≈ 0.17). In addition to the expected steps at V n = nhv/2e, n = 0,1,2,etc., we record an odd sequence of steps at an rf current amplitude I rf /I c ≈ 3.81, and an even sequence of steps at I rf /I c ≈ 3.96. For I rf /I c ≈ 2.22, both even and odd steps are present after the 4th step. The measured rf power and frequency dependences of the step heights are in excellent agreement with the Resistively-Shunted Junction (RSJ) model predictions.