APPLICATION OF A CAVITY PERTURBATION METHOD TO THE MEASUREMENT OF THECOMPLEX MICROWAVE IMPEDANCE OF THIN SUPER-CONDUCTING OR NORMAL CONDUCTING FILMS

We present the application of a cavity perturbation method on thin sup er- or normal conducting films. The sample is placed in the center of the cavity in the maximum of the electric field. We have calculated th e complex microwave impedance of the films with a new approach from th e measured data. The method allows to determine the complex impedance of films with arbitrary thickness. In particular, films with thickness d small compared to the skin depth delta or the London penetration de pth lambda can be measured. Therefore, the impedance of superconductin g films can be measured both in the normal and superconducting state. The method is an important completion to the conventional measurements of the surface resistance which replace one of the cavity walls by th e film. This arrangement is problematic for films with d < lambda or d elta because in this case the field transmits through the film out of the cavity.