Two different thermally activated flux-flow regimes in oxygen-deficient YBa2Cu3O7-x thin films.

Abstract

We have measured the temperature dependence of the longitudinal resistivity of an oxygen-deficient YBa{sub 2}Cu{sub 3}O{sub 6.45} thin film in perpendicular applied magnetic fields {ital B}{lt}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 {ital B}{sub cr}=136.13{sup *}(1{minus}{ital T}/{ital T}{sub {ital c}})/{ital T} (T{sub c}=30.35 K, {ital B}{sub cr} 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{minus}{ital T}/{ital T}{sub {ital c}})ln{ital B}. 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 {ital U}{sub 0}{proportional_to}(1{minus}{ital T}/{ital T}{sub {ital c}}){ital B}{sup {minus}0.46}.