Coherent response of two nearly identical stacked Josephson junctions to mm wave irradiation

Abstract

Abstract Double-barrier Nb/Al-AlO x -Nb/Al-AlO x -Nb tunnel devices with nearly identical Josephson critical current densities of the single junctions have been investigated under influence of the electromagnetic irradiation in the frequency range from 78 to 119 GHz. Odd, even, and fractional Shapiro steps have been observed in the weak-signal regime. The switching of the device to the double gap-sum voltage occurs from even steps, whereas the switching to the single gap-sum voltage has usually been observed from odd steps. The even steps have a much larger amplitude than the odd steps. This behavior is explained as a manifestation of the coherent phase-locking of the two junctions to the applied mm wave signal. Fractional steps may be evidence of a nonlinear interaction between the junctions.