Comparative Description of the Microwave Surface Impedance of Nb, BaKBiO, and YBaCuO

Abstract
Measurements of microwave impedance versus temperature, , in Nb, Ba _0.6K _0.4BiO ₃, and YBa ₂Cu ₃O are given. The measurements were made by the "hot finger" cavity perturbation technique. The electromagnetic properties of Nb are in a good quantitative agreement with the isotropic s-wave BCS theory. This model can also describe the experimental curves in Ba _0.6K _0.4BiO ₃ crystals if the nonhomogeneous broadening of the superconducting transition is properly taken into account. The behavior of the microwave impedance of YBa ₂Cu ₃O disagrees with the isotropic BCS theory. This suggests a strong anisotropy of the order parameter in this compound. Since the controversy between s- and d-wave descriptions of YBa ₂Cu ₃O properties has not been resolved yet, we have checked a simpler strongly anisotropic s-wave model. Calculations by this model have been compared to measurements on YBa ₂Cu ₃O . With due account of the strong-coupling effects, two-band anisotropy, and impurity scattering in YBa ₂Cu ₃O , the agreement between the generalized BCS model and experimental data is surprisingly good.