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Transport properties of ultrathin YBa2Cu3O7- delta layers: Evidence for two-dimensional vortex fluctuations.

We report on the transport properties of ultrathin YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]] (YBCO) layers, and compare the results with predictions of the Ginzburg-Landau Coulomb-gas (GLCG) model for two-dimensional (2D) vortex fluctuations. We find that the normalized flux-flow resistances for several ultrathin YBCO structures collapse onto a single universal curve, as predicted by the model. In addition, the values for the Kosterlitz-Thouless transition temperature [ital T][sub KT] and the Ginzburg-Landau temperature, [ital T][sub [ital c]0], obtained by separate analyses of [ital I]-[ital V] and resistance data within the context of the GLCG model, are in general agreement. Finally, we find that the properties of a series of YBCO/Pr[sub 0.5]Ca[sub 0.5]Ba[sub 2]Cu[sub 3]O[sub 7[minus][delta]] superlattice structures are consistent with the GLCG treatment for anisotropic, 3D layered superconductors.