ANISOTROPIC PROPERTIES OF THE MICROWAVE MAGNETO-DISSIPATION IN HIGH-T-C SUPERCONDUCTING FILMS

we report measurements of the magnetic field dependent microwave dissipation at 21 GHz in Bi2Sr2CaCu2O8+x epitaxial films and in oriented YBa2Cu3O7-delta films, by means of the end-wall-replacement cavity technique. The power reflected by the tuned cavity is measured as a function of the external magnetic field H and of the angle theta between H and the (a, b) planes, at fixed temperatures. In Bi2Sr2CaCu2O8+x, apart from a narrow (Delta theta approximate to 0.5 degrees) angular range close to the parallel orientation, the strongly anisotropic dissipation exhibits striking scaling properties: all the data at fixed temperature can be made to collapse on a single curve with a simple angular scaling of the magnetic field: H --> H/f(theta). It is found that the anisotropy ratio f(0 degrees)/f(90 degrees) increases with temperature. It is shown that the scaling function f(theta) and its temperature evolution are in agreement with the model for a quasi-2D superconductor. In YBa2Cu3O7-delta the presence of the flux-flow dissipation prevents the application of a scaling procedure, and a specific model must be applied. We obtain that the anisotropy is almost constant with the temperature, as predicted for a three-dimensional, anisotropic superconductor.