Identification and modeling of microwave loss mechanisms in YBa/sub 2/Cu/sub 3/O/sub 7-X/

It has been proposed that the nonlinear microwave response of YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films is due to small- or large-angle grain boundaries which behave as weak links. To identify dominant loss mechanisms in the measured response of stripline resonators, the relative change in surface reactance and resistance, r=/spl Delta/X/sub s///spl Delta/R/sub s/, is used as a characteristic signature for the different mechanisms. We show that Meissner state and flux flow losses can be differentiated from hysteresis loss for a variety of films across a range of microwave powers, frequencies, and temperatures. In addition, a coupled-grain/RSJ model has been augmented to include Josephson currents up to and beyond the critical currents J/sub cj/ of the weak links. The model can account for a distribution of grain and boundary properties including I/sub c/R/sub n/ products, junction capacitances, and grain dimensions. The r-values predicted by the model are discussed and compared to measurements.