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

We report on measurements of the angular dependence of the irreversibility temperature Tirr(θ) in YBa2Cu3O7-δ thin films, defined by the onset of a third-harmonic signal and measured by a miniature Hall probe. From the functional form of Tirr(θ) 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. copyright 1996 The American Physical Society