An anomalous weak link model for microwave surface impedance of YBa2Cu3O7-δ films in a weak dc magnetic field

By incorporating an anomalous weak link (AWL), the coupled-grain model is modified to describe the anomalous decrease of microwave surface impedance (Z(s)) of YBa2Cu3O7-delta thin films in a weak dc magnetic field (B-dc) The critical current density of the AWL has a minimum at zero B-dc, a linear dependence on B-dc for low B-dc and a saturation at high B-dc, unlike a conventional weak link. Using this model, the field dependences of surface resistance are calculated at different rf input powers and resonant modes. The dimensionless parameters r(H), defined as the ratio of the change of surface resistance and reactance, are also computed. The simulation results agree with the measurement qualitatively. We believe that the anomalous Z(s) behaviour may originate from the AWL.