Shapiro Steps in the Absence of Microwave Radiation

The current-voltage I-V characteristics of a 1.2 \mum long Au nanowire contacted by superconducting electrodes were studied in details. Interestingly, the I-V curves over a wide range of temperatures display multiple steps at voltages V = (m/n)(V0/2e) in the absence of microwave radiation, where m, n are integer numbers and V0 ~ 92 \muV. We posit that these steps are the subharmonic Shapiro steps due to the interplay of the ac Josephson current and a minigap in the Au nanowire induced by superconducting electrodes via the proximity effect.