LENGTH-SCALE-DEPENDENT VORTEX-ANTIVORTEX UNBINDING IN EPITAXIAL BI2SR2CACU2O8+DELTA FILMS

The supercurrent transport properties of epitaxial Bi2Sr2CaCu2O8+delta films in zero applied magnetic field were investigated in a temperatu re interval of approximate to 20 K below the mean-held critical temper ature T-c0. The modification of the shape of the I-V curves observed b y varying the temperature was explained in terms of vortex-fluctuation -induced layer decoupling and vortex-antivortex unbinding, revealing a strong probing-length dependence. The change of the effective dimensi onality of thermally excited vortices involved in the dissipation proc ess leads to the appearance of a few characteristic regions in the cur rent-temperature diagram. Above a temperature value T<T-c0, the super conducting layers are decoupled, as predicted by Monte Carlo simulatio ns and renormalization-group analyses. In this region, the resistivity exhibits two-dimensional (2D) behavior corresponding to the supercond ucting (CuO2)(2) layers (2D-layer behavior. However, the resistive tra nsition seems to be mainly related to the 2D behavior at the film leve l. In the sensitivity window of our measurements, finite resistance in the limit of small transport currents was detected to occur above a t emperature value T-c. <T, through the dissociation of vortex-string-a ntivortex-string pairs. By decreasing the temperature and/or by increa sing the transport current, the I-V curves in the double logarithmic p lot show a clear downward curvature. This can be described in terms of current-induced quasi-2D vortex pair unbinding, with a nonzero critic al-current density resulting from the interlayer Josephson coupling. A t even lower temperatures and/or higher transport currents, the I-V cu rves exhibit a crossover from quasi-2D to 2D-layer behavior, due to th e decrease of the probing length below the Josephson length, where the interlayer Josephson coupling becomes irrelevant. The temperature dep endence of the 2D I-V exponent is in good agreement with recent Langev in simulations of the Coulomb gas model, revealing an anomalous diffus ion of vortex fluctuations.