Surface impedance measurements and calculations according to the Mattis-Bardeen theory on high-Tc superconductors

Surface impedance measurements made on YBa₂Cu₃ O_7-x (YBCO) films are reported and compared with the exact solution of the Mattis-Bardeen theory applied to strong-coupling superconductors. Epitaxial films of YBCO were grown by the coevaporation of Y, BaF₂, and Cu and ex-situ wet O₂ annealing. The films were deposited at substrate temperatures ranging from 200°C to 820°C. X-ray diffraction shows that the films grown in the temperature range 720°C-780°C are epitaxial with the c -axis perpendicular to the substrate surface. Surface impedances of the samples were measured using a cylindrical copper cavity at 66.8 GHz. It is demonstrated that the value of the surface resistance can be considerably reduced on c-axis-oriented epitaxial films with correlated a-b orientation across the whole surface of the film. The calculation of the surface resistance using the Mattis-Bardeen theory is found to explain the experimental behavior with Bardeen-Cooper-Schrieffer (BCS) energy gap value