Current-step-like structure in a YBCO grain boundary Josephson junction coupled to a gigahertz signal

Abstract

We demonstrate current-step-like structures observed in a YBa2Cu3O7−δ (YBCO) grain boundary Josephson junction (GBJJ) coupled to a gigahertz (GHz) signal. It is well known that a microwave-current-driven Josephson junction does not exhibit clear Shapiro steps under a low frequency condition of f << IcRn/ϕ0, where f, Ic, Rn, and ϕ0 are the microwave frequency, the critical current of the junction, the normal resistance of the junction, and a flux quantum, respectively. Instead of Shapiro steps, however, we often observe an anomalous upturn in the current-voltage curve of YBCO GBJJ's coupled to a GHz signal. We refer to the upturn as a current-step-like structure. Numerical analysis using an rf-field-driven two-junction superconducting quantum interference device (SQUID) model indicates that the envelope of small Shapiro steps, including the effects of microwave-induced motion of flux quanta, appears as a current-step-like structure.