Microwave dynamics of YBCO bi-epitaxial Josephson structures

The processes of interaction of microwaves (frequency View the MathML source) with a single high-Tc superconducting YBa2Cu3Ox (YBCO) bi-epitaxial grain-boundary junction and with an array of two junctions connected in series, have been investigated experimentally at temperatures T = 4.2− 77 K, and magnetic fields B = −0.4 to +0.4 mT. The results obtained experimentally were used as the input data for computer simulation which confirms the hypothesis that the current through the grain-boundary junction is transported by a parallel array of lumped Josephson junctions. This model for a grain-boundary junction explains the observed unusual magnetic field dependence of the critical current Ic(B), and the deviations of dynamic processes from the predictions of the well known resistively shunted junction (RSJ) model: the existence of large amplitude subharmonic (View the MathML source) Shapiro steps, as well as the subharmonic detector response at weak magnetic fields φ <φ0. We also discuss the spectrum of this junction in connection with the half-integer Shapiro steps, and experimentally observed microwave field induced frequency synchronization of two series connected bi-epitaxial YBCO junctions.