Frequency dependence of the AC-sheet conductivity in thin YBa2Cu3O7-delta films

The frequency dependence of the AC-sheet conductivity near the superconducting transition temperature, T-c, has been measured for two films of YBa2Cu3O7-delta: One ultra-thin film of thickness 25 Angstrom and one film of thickness 4000 Angstrom. The results are compared to the predicted behaviour at a Kosterlitz-Thouless (KT) transition, Neither of the samples enters a KT critical region. The thicker film appears three dimensional in the vicinity of T-c, and excitations other than vortex-anti-vortex pairs govern the transition. The thinner sample is two dimensional in the entire temperature region, but the KT transition is hampered by effects of finite penetration depth, finite sample size and/or sample inhomogeneities. The magnetic noise power density was also measured and compared to the AC-conductivity data, showing that the fluctuation-dissipation theorem is valid.