Subharmonic Shapiro steps in Josephson-junction arrays

Abstract
Subharmonic Shapiro step structure has been observed in the current-voltage characteristics of 300×300 Nb-Au-Nb superconducting arrays. This subharmonic structure does not appear in the current-voltage characteristics of individual junctions, but appears to reflect a unique dynamical mode of the coupled system. A qualitative model is proposed that attributes the structure to the dynamical response of quantized loops of flux that are created by the antisymmetric self-magnetic field of the array bias current. Application of an external asymmetric magnetic field modifies the structure, which lends strong support to our model.