Two-dimensional vortex and phase fluctuations from current-voltage characteristics of Bi2Sr2CaCu2O8+x films with various oxygen contents

Current-voltage characteristics of 2212-phase epitaxial Bi2Sr2CaCu2O8+x films have been measured in a temperature region about 8 K wide immediately below Tc. It is shown that the power law V=A Ia valid for two-dimensional (2D) systems does not correctly describe the data, and interlayer coupling must be taken into account. The 2D approximation still remains valid at high currents, so that a crossover from 2D to 3D behavior is observed as the current is reduced. Furthermore, annealing treatments in redox atmospheres change the interlayer coupling, therefore leading to an increase of the critical current density and a decrease of the anisotropy factor with increasing oxygen content in the film. These effects can be quantitatively evaluated fitting the I-V curves with a theory due to Jensen and Minnhagen valid for quasi-2D systems. Not too far from Tc, however, an additional dissipation appears besides the Jensen-Minnhagen contribution in the low-voltage region. We propose to explain this in terms of fluctuations of the phase of the order parameter. The calculated effect is in reasonable agreement with the experimental data. As the oxygen content in the sample is varied the relative weight of the two contributions changes in qualitative agreement with the proposed fluctuation origin of the additional dissipation at low voltages.