Fractal structure near the percolation threshold for YBa2Cu3O7 epitaxial films

Inhomogeneity of the properties of YBa2Cu3O7 epitaxial c-oriented films has been investigated. Using a low - temperature SEM the lateral distribution of local critical temperature has been determined with spatial resolution of 2 mu m. The technique provides a direct observation of the percolative character of the superconducting transition on a microbridge scale. It is shown for the first time that near the percolation threshold the current channel has a very rough, irregular configuration and the fractal dimensionality of the superconducting cluster is estimated as 1.55. The magnetic field imaging of samples is carried out and a quantitative correlation between T-c-map and the map of critical density of shielding currents is observed. The transition curves R(T) over a wide range of resistances and bias currents are described in terms of the network model. The model takes into account both the spatial distribution of T-c and the presence of random weak links with a wide distribution of their critical currents.