Origin of the irreversibility line in thinYBa2Cu3O7−δfilms with and without columnar defects

We report on measurements of the angular dependence of the irreversibility temperature {ital T}{sub irr}({theta}) in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} thin films, defined by the onset of a third-harmonic signal and measured by a miniature Hall probe. From the functional form of {ital T}{sub 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. {copyright} {ital 1996 The American Physical Society.}