2 DIFFERENT THERMALLY ACTIVATED FLUX-FLOW REGIMES IN OXYGEN-DEFICIENT YBA2CU3O7-X THIN-FILMS

We have measured the temperature dependence of the longitudinal resistivity of an oxygen-deficient YBa2Cu3O6.45 thin film in perpendicular applied magnetic fields B<12 T. A crossover from a three-dimensional (3D) flux-line liquid to a quasi-2D liquid of vortices is identified which is characterized by a crossover line Bcr=136.13*(1-T/Tc)/T (Tc=30.35 K, Bcr in T). In both 3D and 2D regimes, the dissipations are thermally activated. Above the crossover temperatures, the vortices are in a quasi-2D liquid state and the dissipation is dominated by the motion of edge dislocation pairs or vortex cutting and the activation energy is proportional to (1-T/Tc)lnB. Below the crossover temperatures and above the irreversibility line, the vortices are in a 3D line liquid state, and the dissipation is governed by plastic deformation of the vortices through double kink nucleations in the vortices with activation energies U0(1-T/Tc)B-0.46. © 1995 The American Physical Society.