WOLTE 2
Proceedings of the Second European Workshop on Low Temperature Electronics

J. Phys. IV France 06 (1996) C3-259-C3-264
DOI: 10.1051/jp4:1996339

Fractal Structure Near the Percolation Threshold for YBa₂Cu₃O₇ Epitaxial Films

M. Baziljevich¹, A.V. Bobyl², H. Bratsberg¹, R. Deltour³, M.E. Gaevski⁴, Yu.M. Galperin¹, V. Gasumyants³, T.H. Johansen¹, I.A. Khrebtov⁵, V.N. Leonov⁵, D.V. Shantsev² and R.A. Suris<sup>4

1</sup>  University of Oslo, 0316 Oslo 3, Norway
²  St. Petersburg State Technical University, St. Petersburg 195251, Russia
³  Université Libre de Bruxelles, 1050 Brusseles, Belgium
⁴  A.I. Ioffe Physical-Technical Institute, St. Petersburg 194021, Russia
⁵  S.I. Vavilov State Optical Institute, St. Petersburg 199034, Russia

Abstract
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 µm. 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.

© EDP Sciences 1996