2-DIMENSIONAL FLUCTUATIONS CLOSE TO THE ZERO-FIELD TRANSITION OF BI2SR2CACU2O8

The ac conductivity along the CuO2 layers of epitaxial films and singl e crystals of thicknesses 0.2 mu m less than or equal to L-z less than or equal to 130 mu m reveals clear signatures for two-dimensional ord ering. In particular, at T-c the screening length assumes the universa l value L-z Lambda(Tc)/2 nu(s) predicted by the Kosterlitz-Thouless th eory, which indicates that binding of long vortex strings nucleates lo ng-range superconductivity in the present samples containing up to 10( 5) CuO2 layers. Several details of the order-parameter dynamics remain unexplained.