FRACTIONAL GIANT SHAPIRO STEPS IN 2D JOSEPHSON ARRAYS

Radio-frequency signals were applied to 1000 by 1000 arrays of SNS Josephson junctions and giant Shapiro steps were observed at voltages 1000 times that expected for a single junction, Vn= nNhv/2e, where N=1000 is the number of junctions in the direction of applied current. For certain perpendicular magnetic fields the field-induced vortices are strongly commensurate with the array and are arranged in superlattices with qxq unit cells. In the presence of these strongly commensurate fields novel fractional giant steps were observed at voltages, Vn =n(N/q)hv/2e, directly related to q. The locking of the motion of the vortex superlattice with the applied rf current is used to explain these novel steps. The possibility of subharmonic fractional steps and preliminary measurements on disordered arrays is discussed. © 1990, Elsevier Science Publishers B.V. (North-Holland). All rights reserved.