Abstract
A.c. susceptibility measurements have been carried out on Nb 3.25(Al 0.8Ge 0.2) and V 3Si in a wire, powder or compact form and on (RE)Ba 2Cu 3O 7−δ powdeers and sintered compact materials. They show the dependence of the complex susceptibilityχ = χ′ + iχ″ on the surface volume ratio of the grains and on the contact resistance between the grains (i.e. the ability of carrying supercurrent). In this way, below the bulk transition (Tc bulk), a non-bulk superconducting transition can occur with a strong dependence of χ′ and χ″ on the a.c. magnetic field amplitude. This typical behaviour is explained in terms of weakly connected superconducting loops, and Josephson interactions between grains in granular superconductors.
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