Subharmonic Shapiro steps in high-Tc Josephson junctions

We studied the response of high-Tc biepitaxial grain boundary junctions to 100 GHz radiation in the presence of a magnetic field. Integer as well as subharmonic constant voltage steps are observed, even at one-fifth of the voltage separation between integer ~Shapiro! steps. Our results indicate that this behavior is due to the synchronized motion of Josephson vortices along the junction. We show that this effect is directly related to the width of the junction, and not to an array of weaker regions in the barrier. © 1995 American Institute of Physics.