FRACTAL STRUCTURE NEAR THE PERCOLATION-THRESHOLD FOR YBA2CU3O7 EPITAXIAL-FILMS

Inhomogeneity of the properties of YBa2Cu3O7 epitaxial c-oriented film s has been investigated. Using a low - temperature SEM the lateral dis tribution of local critical temperature has been determined with spati al resolution of 2 mu m. The technique provides a direct observation o f the percolative character of the superconducting transition on a mic robridge scale. It is shown for the first time that near the percolati on threshold the current channel has a very rough, irregular configura tion and the fractal dimensionality of the superconducting cluster is estimated as 1.55. The magnetic field imaging of samples is carried ou t and a quantitative correlation between T-c-map and the map of critic al 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 spa tial distribution of T-c and the presence of random weak links with a wide distribution of their critical currents.