AC-susceptibility of YBaCuO and YBaCuO:Ag ceramics in terms of the percolation theory

At least three factors determine the magnitude of alternating magnetic flux which penetrates into inhomogeneous superconducting (spcd) material: 1. Geometrical parameters of spcd part of the material (fraction of the spcd volume v, correlation length, etc.) 2. Parameters of the Josephson network (coupling energy, normal resistance of the contacts, etc.) 3. Cooperative phenomena which control phase coherence of the spcd wave function; they depend on relations between different lengths: grain size d, spcd coherence length ξ, spcd penetration length λ, etc. To study comparative role of these factors, we measured ac-susceptibility of ceramic YBaCuO and YBaCuO:Ag with oxygen deficit. Grains in such samples have slightly different oxygen content and, as a result, different Tc. This allows to alter the first factor, volume v, by changing T. By increasing the ac amplitude h the spcd contacts are destroyed. This allows to separate contributions from the grains themselves and from spcd clusters which embrace normal regions. Comparison of the samples with and without Ag clarifies the role of contacts parameters, as it is known that Ag in YBaCuO concentrates in the grain boundaries. Finally, data analysis on the classical percolation basis brings light to cooperative phenomena. (orig.)