Nonlinear surface impedance for YBa2Cu3O7-x thin films: Measurements and a coupled-grain model.

We present measurements of the surface impedance as a function of frequency (1-17 GHz), temperature (4.2-91 K), and peak rf magnetic field (0stripline-resonator technique. The lowest surface resistance at 1.5 GHz was 15 μΩ at 77 K and 3 μΩ at 4.3 K. The results for the low- and intermediate-field regions (H rf <50 Oe at 77 K) are explained by a coupled-grain model, which treats the film as a network of superconducting grains connected by grain boundaries acting as resistively shunted Josephson junctions. Quantitative agreement has been obtained between the model and the measurements