Fractal Structure Near the Percolation Threshold for YBa2Cu3O7 Epitaxial Films

Inhomogeneity of the properties of YBcr2Cu307 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 pm. 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 Tcmap and the map of critical density of shielding currents is observed. The transition curves H(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 Tc and the presence of random weak links with a wide distribution of their critical currents.