Effect of anisotropy on the vortex liquid dissipation in YBa2Cu3O7-δ thin films

Transport measurements have been used to investigate the vortex dynamics in the mixed state on high quality deoxygenated YBa2Cu3O7-delta thin films, in magnetic fields up to 8 T. Epitaxial YBa2Cu3O7-delta thin films, grown by high pressure de sputtering, were deoxygenated in situ following a stability line of the pressure-temperature phase diagram. We show that varying the oxygen content, the dynamics of the vortex liquid phase reveal a crossover from a densely entangled liquid to a decoupled state. At small oxygen deficiencies the activation energy for vortex motion is found to depend as 1/H on the applied magnetic field, suggesting a densely entangled liquid. Strongly deoxygenated samples however, show a crossover from an entangled liquid, with a 1/H-0.5 dependence of the activation energy in magnetic fields up to 2 T, to a decoupled state, characterized by a logarithmic dependence for higher fields. (C) 2001 Elsevier Science B.V. All rights reserved.