Effects of the Variable Lorentz Force on the Critical Current in Anisotropic Superconducting Thin Films

When a current is applied perpendicular to the vortex lattice (VL), Lorentz force may cause the VL to drift and flux-flow dissipation is observed. When the current is parallel to the applied magnetic field in a force-free (FF) configuration, a dissipation is also observed but at higher values of applied current. It has been suggested that pinning as well as free surfaces play an important role in the stabilization of the VL in the FF configuration. In thin YBa₂Cu₃O₇ films, FF configurations can be obtained when Hparab with the current flowing parallel to the ab-planes. In this work we study the influence of thickness, growth method and pinning centers on the dissipation mechanism at variable Lorentz force and FF configurations. Comparisons of experiments done at maximum and variable Lorentz force show that there are two pinning regimes when the field is rotated in these configurations; one consistent with only a decrease in the applied force, indicated by the overlap of the power law exponent of the current-voltage curves as the field is rotated toward the ab-planes, and another very close to the ab-planes, where the dissipation characteristics change.