FIELD-DEPENDENT CROSSOVER IN THE VORTEX RESPONSE AT MICROWAVE-FREQUENCIES IN YBA2CU3O7-DELTA FILMS

We observe anomalous magnetic field dependence of the microwave vortex response of epitaxial YBa2Cu3O7-delta films, which we identify as a crossover from flux pinning to flux flow, occurring near a characteristic field H*(T). We show that the presence of the crossover depends only on adequate softening of the pinning potential U(T,H) (i.e., it is independent of the nature of the vortex transition). However, the data are in good agreement with a thermal activation model in which U(T,H) = A (T)/H. A separate analysis of the high-field, high-frequency superconducting transition yields a measure of the upper critical field H(c2).