Investigation of double-barrier Nb-Al-AlO/sub 2/-Al-AlO/sub x/-(Al-)Nb junctions under high-frequency irradiation

We have studied experimentally the current-voltage characteristics of double-barrier Nb-Al-AlO/sub x/-Al-AlO/sub x/-(Al-)Nb junctions exposed to microwave radiation at 4.2 K. Both integer and fractional Shapiro steps were observed. A complicated behavior of the step heights was observed as a function of both the microwave power and an applied magnetic field. Reasonable agreement with the RSJ model was obtained only for the steps 0 to 2 observed in current-voltage characteristics of the junctions with a "clean" middle Al layer. A deviation from RSJ-like behavior was found for higher-order integer steps and fractional steps. Devices with a "dirty" middle Al layer displayed more significant deviation from RSJ-like behavior. Step heights vs. magnetic field dependences in some cases were found to be qualitatively different from the field dependence of the dc Josephson current.