Origin of the irreversibility line in thin YBa2Cu3O7- delta films with and without columnar defects.

We report on measurements of the angular dependence of the irreversibility temperature Tirr (�) in Y Ba2Cu3O7 � thin films, defined by the onset of a third harmonic signal and measured by a miniature Hall probe. From the functional form of Tirr (�) 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 films the irreversibility temperature is determined by the trapping angle.