ORIGIN OF THE IRREVERSIBILITY LINE IN THIN YBA2CU3O7-DELTA FILMS WITHAND WITHOUT COLUMNAR DEFECTS

We report on measurements of the angular dependence of the irreversibi lity temperature T-irr(theta) in YB2Cu3O7-delta thin films, defined by the onset of a third-harmonic signal and measured by a miniature Hall probe. From the functional form of T-irr(theta) we conclude that the origin of the irreversibility line in unirradiated films is a dynamic crossover from an unpinned to a pinned vortex liquid. In irradiated fi lms the irreversibility temperature is determined by the trapping angl e.