Surface impedance measurements and calculations according to the Mattis-Bardeen theory on high-T/sub c/ superconductors

摘要

Surface impedance measurements made on YBa/sub 2/Cu/sub 3/O/sub 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, BaF/sub 2/, and Cu and ex-situ wet O/sub 2/ annealing. The films were deposited at substrate temperatures ranging from 200 degrees C to 820 degrees C. X-ray diffraction shows that the films grown in the temperature range 720 degrees C-780 degrees 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.