INPLANE SURFACE IMPEDANCE OF EPITAXIAL YBA2CU3O7-DELTA FILMS - COMPARISON OF EXPERIMENTAL-DATA TAKEN AT 87 GHZ WITH D-WAVE AND S-WAVE MODELS OF SUPERCONDUCTIVITY

We have measured the temperature dependence of both the surface resist ance and the change of the penetration depth in two optimized epitaxia l c-axis oriented YBa2Cu3O7-delta (YBCO) films at 87 GHz by incorporat ing each film as an end plate in a cylindrical copper cavity. A high f requency is used in order to increase losses in the superconducting sa mples relative to the losses in the copper cavity. It is found that ou r measuring frequency is of a magnitude comparable to the relevant low -temperature scattering rates, so that the real part of the conductivi ty would be expected to display significant frequency dependence. The two films investigated were both 350 nm thick, but prepared by differe nt techniques. The experimental results are compared to weak-coupling d- and s-wave models of superconductivity which incorporate both inela stic and elastic scattering, with the latter forming a small part of t he total scattering. The sizable surface resistance at low temperature s and the approximately linear temperature variation can be accounted for without subtracting an extrinsic residual surface resistance, if d - or anisotropic s-wave order parameters with nearly vanishing Fermi s urface averages and scattering phase shifts close to 0.4 pi are assume d. Large low-temperature losses are obtained theoretically in spite of the fact that order parameter amplitudes must be in the range of 2 De lta(0)(0)/k(B)T(c)=6.0-7.5, considerably larger than the corresponding weak-coupling values, in order to describe the data at higher tempera tures. When inelastic scattering is represented by a phenomenological temperature-dependent scattering rate, a quantitative fit to the exper imental data for both the surface resistance and the penetration depth of YBCO over the whole measured temperature range from 4.2 to 145 K c an be obtained within a single model. Some discrepancy between theory and experiment remains near the transition temperature where fluctuati on effects, not treated in this paper, are clearly visible. While very different parameter sets can be found that would fit the real part of the conductivity, having to explain both penetration depth and surfac e resistance puts severe constraints on the available parameter space. A description of the inelastic scattering on the basis of spin fluctu ation exchange within the nested Fermi-liquid model with full frequenc y dependence taken into account still gives reasonable fits to the dat a, even though Only a single parameter, fixed by the normal-state resi stivity, is involved. For s-wave states, whose Fermi surface average i s a sizable fraction of the order-parameter amplitude, scattering rate s drop well below the experimental frequency at sufficiently low tempe ratures for the whole range of scattering phase shifts. Thermally exci ted quasiparticles still present then act as a nearly ideally conducti ng system which results in losses too low to be compatible with the ex perimental observations.