Two-Dimensional Fluctuations Close to the Zero-Field Transition of Bi_2Sr_2CaCu_2O_8

Abstract

The ac-conductivity along the CuO${}_2$-layers of epitaxial films and single crystals of thicknesses 0.2$\mu$m$\le L_z\le 130\mu$m reveals clear signatures for 2D-ordering. In particular, at $T_c$ the screening length assumes the universal value $L_z{\Lambda }_{T_c}/2{\nu }_s$ predicted by the Kosterlitz-Thouless theory, which indicates that binding of long vortex strings nucleates long-range superconductivity in the present samples containing up to $10^5$ CuO${}_2$-layers. Several details of the order-parameter dynamics remain unexplained.

4 pages LaTeX (REVTEX)and 4 separate eps-figures, accepted for publication in Phys.Rev.B