Kosterlitz-Thouless transition and conductivity fluctuations in Y-Ba-Cu-O thin films

Kosterlitz-Thouless properties of c-axis-oriented high-${\mathit{T}}_{\mathit{c}}$ superconducting ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ thin films were examined. The current-voltage characteristics over a wide current range obeyed a power-law relationship. The exponent α(T) showed a ‘‘universal jump’’ at ${\mathit{T}}_{\mathit{c}}$. The exponential inverse-square-root reduced-temperature dependence of the resistance above ${\mathit{T}}_{\mathit{c}}$ was consistent with a two-dimensional (2D) behavior of the thin films. Excess conductivity due to thermal fluctuations of the order parameter above the mean-field transition temperature ${\mathit{T}}_{\mathit{c}0}$ was also measured. However, the paraconductivity showed a 1D-like behavior at temperatures close to ${\mathit{T}}_{\mathit{c}0}$ and further complications at higher temperatures. This 1D-like behavior could be the result of imperfections of the dimensionality of 2 in the polycrystalline sample.