Surface impedance of epitaxial YBCO thin films ? Comparison of experimental data with a d-wave theory of superconductivity

The surface impedance of two 350 nm thick YBCO films was measured for temperatures between 4.2 K and 150 K in a copper cavity at 87 GHz. Both films, one grown by electron beam evaporation on MgO, the other one by high oxygen-pressure dc sputtering from a stoichiometric target on LaAlO sub(3), provide critical temperatures of about 91 K, low residual surface resistances of R sub(s)(4.2 K) < 1 m omega and low specific resistivities in the normal state of rho (100 K) < 85 mu omega cm. The experimental data obtained on these two films are compared to a d-wave model of superconductivity which incorporates elastic and inelastic scattering. Good agreement between theory and the experimental results for both the surface resistance and the penetration depth in the whole temperature range is achieved for scattering phase shifts near 0.4 pi and order parameter amplitudes in the range of 2 delta sub(0)(0)/k sub(B)T sub(c) identical with 6.0-7.5 without subtracting an extrinsic residual surface resistance.