MODULATION OF LOW-FREQUENCY SHAPIRO STEPS OF SUPERCONDUCTING PROXIMITY-EFFECT JUNCTIONS

We observe a modulation of Shapiro step heights in superconducting Nb-Au-Nb (SNS) Josephson junctions at an rf frequency v = 20 MHz (hv/2eRIc ≈ 0.17). In addition to the expected steps at Vn = nhv/2e, n = 0,1,2,etc., we record an odd sequence of steps at an rf current amplitude Irf/Ic ≈ 3.81, and an even sequence of steps at Irf/Ic ≈ 3.96. For Irf/Ic ≈ 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. © 1990, Elsevier Science Publishers B.V. (North-Holland). All rights reserved.