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

Abstract

By incorporating an anomalous weak link (AWL), the coupled-grain model is modified to describe the anomalous decrease of microwave surface impedance (Zs) of YBa2Cu3O7-δ thin films in a weak dc magnetic field (Bdc). The critical current density of the AWL has a minimum at zero Bdc, a linear dependence on Bdc for low Bdc and a saturation at high Bdc, 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 rH, 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 Zs behaviour may originate from the AWL.