Modeling the nonlinear surface impedance of high-Tc thin films

To explore the idea that weak links are responsible for power dependence, we have measured and modeled the microwave impedance of fabricated Josephson-junctions, in which we have observed evidence for Josephson-vortex creation and annihilation by the microwave currents. We believe that we have observed long-junction effects in fabricated high-T_c Josephson-junctions and that these are the first observation of microwave-frequency vortex creation. To explain the nonlinear surface impedance of thin films of the high-T_c materials, an extended-coupled-grain model is introduced and discussed. The model incorporates a series array of Josephson-junction defects with a distribution of I_cR_n products. Comparison with experiments shows good qualitative agreement.