DISSIPATIVE FLUX MOTION IN EPITAXIAL YBA2CU3O7-DELTA AND BI2SR2CACU2O8+X FILMS

We have performed a comparative study of the dissipative flux motion in the anisotropic and the very anisotropic high temperature superconductors YBa2Cu3O7-delta and Bi2Sr2CaCu2O8+x, respectively, by AC susceptibility measurements in an applied DC magnetic field between 0.01 and 7 T. The limit of small current densities (10(-4) < J < 1 A/cm2) was investigated by using small amplitudes of the driving AC field. For the very anisotropic Bi2Sr2CaCu2O8+x superconductor the thermally activated motion of two-dimensional vortex lattice defects, which we refer to as plastic flux creep, is found to prevail over the motion of elastically correlated flux bundles, or elastic creep. The magnetic field dependence of the activation barrier and the irreversibility lines are found in good agreement with plastic flux creep theory. For the less anisotropic superconductor YBa2Cu3O7-delta the flux motion is dominated by elastic flux creep. For this material the detailed measurement of the frequency and amplitude dependence of the AC susceptibility transition gives clear evidence for a finite temperature phase transition to a vortex glass state. The relevant critical exponents and the magnetic field dependent glass temperature are derived. Our results demonstrate the influence of the anisotropy of the superconducting material on the dissipative flux motion.