Laser probing of high-T/sub c/ superconducting thin films

The spatial inhomogeneity of the electrical characteristics of high-T/sub c/ thin films was studied by optical laser probing. It was shown that the characteristic length L of inhomogeneous current distribution in high-T/sub c/ films may be much larger than the grain size a; for example, in polycrystalline YBa/sub 2/Cu/sub 3/O/sub 7-x/ film with critical current densities j/sub c/(4.2 K) approximately 10/sup 3/-10/sup 4/ A/cm/sup 2/ and grain size a approximately 1 mu m, the characteristic length L was equal to approximately 15 mu m at T>T/sub c/ and approximately 100 mu m at T<or approximately=T/sub c/. The large values of L are related to large-scale percolation in the system of intergrain Josephson junctions with very wide distribution of critical currents I/sub ci/ and normal-state resistances R/sub ni/.

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