Coupled-grain/RSJ series array for modeling of nonlinear microwave surface impedance of YBCO thin films

The nonlinear microwave impedance of YBCO thin films may be due to Josephson-junction-like defects. In this work, high-T/sub c/ thin films are modeled as a series of ideal superconducting grains coupled by resistively shunted Josephson junctions that simulate the defects. The model calculates the surface resistance and reactance as a function of microwave current. Comparisons with experimental results from YBCO thin-film stripline resonators show good quantitative agreement for resistance at frequencies in the range 1-6 GHz and at several temperatures. Long junction effects are proposed to explain deviations between measured and computed results above 6 GHz. We have also modeled a phase-shifting device consisting of 300 high-T/sub c/ Josephson junctions in series along a microstrip transmission line. Computed results are in good quantitative agreement with the measured device phase shift versus microwave input power. Similarities in the power dependencies of the series array of junctions and the patterned film resonators are discussed.