Abstract
Transport measurements have been used to investigate the vortex dynamics in the mixed state on high quality deoxygenated YBa 2Cu 3O 7− δ thin films, in magnetic fields up to 8 T. Epitaxial YBa 2Cu 3O 7− δ thin films, grown by high pressure dc 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.
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