Dynamical properties of type-II superconductors: An experimental study

We report on surface-resistance measurements of bulk dirty type-II superconductors in the principal orientations between the static magnetic field H, the microwave electric field, and the surface of the sample. When the specimen is in the mixed state, we compare our results with those recently published by Pedersen et al. and we emphasize the difficulties inherent in obtaining the conductivity at Hc2. When H is parallel to the surface of the sample, the conductivity of the surface sheath is strongly anisotropic. The anisotropy can easily be measured by the ratio of the slopes of the surface resistance in the vicinity of Hc3 in the two orientations where the microwave electric field is parallel or perpendicular to H. We find, as calculated by Thompson, that this anisotropy is temperature independent within a large range of temperature, and equal to 1 at Tc. These results are in agreement with the formalism of the fluctuations of the order parameter which was first developed by Caroli and Maki. A careful study shows that the anisotropy has a very sharp peak, higher than 1, in the vicinity of Tc, depending on the κ of the sample. The temperature dependence of the anisotropy, as previously reported by Maki and Fischer, can be explained by the bad state of the surface of their samples, or by a misalignment between H and the surface. We also discuss the notion of "nascent vortices" used as an alternative explanation for the anisotropic conductivity of the surface sheath. Finally, we give some results on the angular dependence of the surface impedance.