Evidence for collectively phase-locked Shapiro steps in stacked Nb/AlAlOx/Nb Josephson junctions☆

Abstract When subjected to microwave radiation, the I - V characteristics of stacked Josephson junctions should exhibit interesting new features which are absent in a single junction. Evidence for collectively phase-locked Shapiro steps and coherent phase-locking of fluxon motion to the external microwave field has been found in observations on stacked Nb/AlAlO x /Nb Josephson junctions.