NONLINEAR SURFACE IMPEDANCE FOR YBA2CU3O7-X THIN-FILMS - MEASUREMENTSAND A COUPLED-GRAIN MODEL

We present measurements of the surface impedance as a function of freq uency (1-17 GHz), temperature (4.2-91 K), and peak rf magnetic field ( 0 < H(rf) < 500 Oe) for high-quality epitaxial YBa2Cu3O7-x thin films using a stripline-resonator technique. The lowest surface resistance a t 1.5 GHz was 15 muOMEGA at 77 K and 3 muOMEGA at 4.3 K. The results f or 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 networ k of superconducting grains connected by grain boundaries acting as re sistively shunted Josephson junctions. Quantitative agreement has been obtained between the model and the measurements. The effective juncti on critical current density, grain size, and shunt resistance are used in the model to characterize the films. The high-field region may be explained by flux penetration into the grains but is not modeled in de tail here.