COMPARISON OF PINNING PARAMETERS BETWEEN LOW-T-C SUPERCONDUCTORS AND YBA(2)CU(3)O7-DELTA

ac flux-profile measurements are used to determine the current density j, the reversible penetration depth lambda' of a pinned flux lattice, and the reversible displacement d, which is the distance flux can mov e before being unpinned. Both parameters lambda' and d describe the el astic regime of the interaction between vortices and defects whereas t he current corresponds to the maximum elastic or plastic distortion. T he influence of thermal relaxation on these parameters and on ac measu rements in general is discussed. Anomalous features of the flux profil es observed in various low-T-c superconductors are correlated with the saturation of the current based on plastic deformation of the vortex lattice. A comparison with flux profiles from single-crystalline and m elt-textured YBa2Cu3O7-delta shows the same anomalous feature. This de monstrates the importance of plastic shear also in high-T-c supercondu ctors which seems to be present in the whole magnetic-field regime. Co llective pinning and the related peak effect of the current in V3Si ar e compared with the fishtail effect in YBa2Cu3O7-delta Single crystals . The different magnetic-field dependences of lambda' and d in the reg ion of maximum current point to differences between these observations . Decreasing current correlated with decreasing lambda' in specimens w ith a fishtail are related to a softening of the vortex lattice at fie lds below the maximum current. This anomalous feature is not observed in samples with a current continuously decreasing with magnetic field.