MICROWAVE SURFACE IMPEDANCE OF YBCO THIN-FILMS IN DC APPLIED FIELDS

We measure the microwave surface impedance of YBa2Cu3O7-delta thin fil ms in the mixed state, for films produced using PLD and electron beam co-evaporation techniques. Measurements were made as a function of fie ld (0-4 T) and temperature (4 K - T-C) at 8 GHz and 16 GHz. From these measurements we extract the flux line restoring force constant (kappa (p)), viscosity (eta), and pinning potential well depth (U). In a magn etic field, deviation from an R(s) proportional to omega(2) indicates a small but significant thermal activation of the flux lines. A field dependent kappa(p) is observed for all samples, where kappa(p) proport ional to B--alpha, and alpha>0. Furthermore a variation in the magnitu de and field dependence of kappa(p) is observed between samples, where the largest values of alpha (similar to 0.4) are observed in the samp les with the smallest values of kappa(p).