TRANSPORT CRITICAL CURRENTS AND FLUX PINNING MECHANISMS IN SINGLE-CRYSTALLINE THIN-FILMS OF YBA2CU3O7-DELTA

In order to investigate the anistropy of transport critical current densities J(c) and flux pinning properties, we have measured J(c) for YBa2Cu3O7-delta single-crystalline thin films in magnetic fields H up to 12 T as a function of the angle between H and the crystal axes. The observed angular dependence of J(c) at 77 K and near 1.0 T could be explained by the model of intrinsic flux pinning proposed by Tachiki and Takahashi under the condition of transport current I perpendicular-to H. However, in a high field region near the upper critical field H(c2), the results disagreed with this model. We found that the reduced field H/H(c2) dependence of the volume pinning force F(p) could be represented by F(p) proportional-to (H/H(c2))1/2 (1 - H/H(c2))2 for all configurations for which we measured. The results indicate that both intrinsic and extrinsic pinning centers worked as a surface-type (two-dimensional) pinning. Furthermore, the angular dependence of H(c2) was explained by the effective mass model.