VORTEX UNBINDING AND LAYER DECOUPLING IN EPITAXIAL BI2SR2CA2CU3O10+DELTA FILMS

The current-voltage characteristics of epitaxial Bi2Sr2Ca2Cu3O10+delta films in zero applied magnetic field were investigated in a temperature interval of about 20 K below the mean-field critical temperature T-co = 110.4 K. Despite the large anisotropy (gamma approximate to 200), the data clearly indicate the occurrence of a finite critical-current density, resulting from the Josephson coupling between the (CuO2)(3) layers. By analyzing the shape of the current-voltage characteristics in terms of quasi-two-dimensional vortex unbinding, it was found that this ''intrinsic'' critical-current density vanishes well below T-co, just above the hypothetical Kosterlitz-Thouless transition temperature ( T-KT approximate to 94.8 K). This is in agreement with Monte Carlo simulations, suggesting a vortex-fluctuation-induced layer decoupling in the case of quasi-two-dimensional superconductors. However, close to the layer decoupling temperature, an excess dissipation beyond the quasi-two-dimensional vortex unbinding model appears. This excess dissipation was attributed to the excitation of three-dimensional vortex structures close to the resistive transition.