THE MEISSNER AND SHIELDING EFFECTS OF HIGH-TEMPERATURE OXIDE SUPERCONDUCTORS

The Meissner and shielding effects were investigated systematically on single crystals of (La1-xSrx)2CuO4, YBa2Cu3O7 and Bi2Sr1.8CaCu2O8. Significant differences in the low-temperature Meissner fraction (f(M)) in the same materials observed under various fields and sample sizes, were explained consistently and quantitatively, based on the vortex-pinning model. Extending the model employed, the pinning critical current could be estimated as a function of T from the set of the shielding and Meissner curves. The Meissner and shielding effects in the polycrystalline samples were also measured in a similarly systematic manner and could be interpreted by considering the effective sample size of the polycrystals, showing a cross-over of this quantity from the grain size at high fields to macroscopic sample size at low fields.