Surface impedance studies of the high-Tc oxide superconductors

The authors briefly review results of measurements of the multimeter-wave surface impedance of ceramic, thin-film and single-crystal samples of the high-T_c oxide superconductors. The observed losses and temperature dependences do not agree with BCS (Bardeen-Cooper-Schriefer) theory. The authors discuss recent measurements of single-crystal B₂CaSr₂Cu ₂O_x and the effect of the application of a perpendicular magnetic field. They consider the possibilities of intrinsic and defect mechanisms to explain the observations. In particular, they discuss the effects of weakly coupled granular films on surface impedance. It is concluded that better samples are necessary to resolve the issue of the source of the large values of the surface impedance observed thus far