MICROWAVE DYNAMICS OF YBCO BIEPITAXIAL JOSEPHSON STRUCTURES

The processes of interaction of microwaves (frequency f(c) = 30-55 GHz ) with a single high-T-c 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 obtaine d experimentally were used as the input data for computer simulation w hich confirms the hypothesis that the current through the grain-bounda ry junction is transported by a parallel array of lumped Josephson jun ctions. This model for a grain-boundary junction explains the observed unusual magnetic field dependence of the critical current I-c(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 (n = 1/2, 3/2,...) Shapiro steps, as well as th e subharmonic detector response at weak magnetic fields Phi < Phi(0). We also discuss the spectrum of this junction in connection with the h alf-integer Shapiro steps, and experimentally observed microwave field induced frequency synchronization of two series connected bi-epitaxia l YBCO junctions.