Angular regimes of ac dynamics and pinning in YBa2Cu3O7 crystals with and without columnar defects -: art. no. 214517

The main source of vortex pinning near the solid-liquid transition at different orientations of a low dc field in YBa2Cu3O7 crystals with columnar defects is investigated by means of ac susceptibility measurements. The behavior is compared with that observed in nonirradiated samples. It is found that in a very wide angular region the tracks act as correlated defects and are the prevailing source of pinning. The results rule out the existence of an accommodation angle, determined by the competition between the pinning and elastic energies, in twined crystals: the interplay with natural correlated pinning centers has to be taken into account. The linear and nonlinear dynamic regimes when the field is tilted relative to the defects are analyzed. It is clearly shown that characterization of the dynamic regimes is crucial for the correct interpretation of the angular dependence in the ac response.