Anomalies in nonlinear microwave surface impedance of YBCO thin films on MgO: superconductor versus substrate effects

The nonlinear microwave surface impedances Z/sub s/ = R/sub s/ + jX/sub s/ of three epitaxial YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) thin films deposited by thermal co-evaporation on MgO substrates have been studied by the coplanar resonator technique at 8 GHz in the temperature T range 12 to 60 K in both zero and finite (/spl sim/ 10 mT) DC magnetic fields. All the films exhibited an anomalous decrease in the surface resistance R/sub s/ as a function of microwave magnetic field H/sub rf/ below 20 K. In one of the films, the application of a DC field of 10 mT has also induced an anomalous decrease in the penetration depth /spl lambda/(H/sub rf/). Analysis of the results suggests that the anomalies may be originating both in the film and in the substrate, and ways to distinguish between the two contributions are proposed. The consequences of the observed phenomena for application of YBCO films in microwave devices are discussed.