MICROWAVE LOSSES OF YBCO THIN-FILMS IN HIGH MAGNETIC-FIELDS

A parallel plate resonator technique was employed to investigate chang es in the microwave surface impedance between 20K and 60K, when d.c. m agnetic fields up to 4T were applied parallel to the YBCO film surface . By using a simple model to describe the observed behaviour, we can e xtract values for the vortex viscosity eta and pinning constant kappa( p), as a function of temperature in three sets of films. As a result o f low energy deuterium ion implantation in one of the films, the low f ield behaviour was degraded but at high fields the pinning properties improved quite drastically. A much smaller effect was observed in fiel d dependence of the dc screening current as measured using vibrating s ample magnetometry.