On the origin of the irreversibility line in thin YBaCuO7 films with and without columnar defects
We report on measurements of the angular dependence of the irreversibility temperature $T_{irr}(\theta) $ in $YBa_2Cu_3O_{7-\delta }$ 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 films the irreversibility temperature is determined by the trapping angle.