PEAK EFFECT AND ITS EVOLUTION FROM OXYGEN DEFICIENCY IN YBA2CU3O7-DELTA SINGLE-CRYSTALS

The current density j in bulk YBa2Cu3O7-delta frequently shows a maxim um at fields far above the self-field. The responsible defect structur e for this peak effect (PE) an small clusters of oxygen vacancies, imp urity atoms, or dopants. The current density caused by these defects i s studied during the evolution of the PE. Very pure, twin-free crystal s without a j(B) peak after high-pressure oxidation were subsequently oxygen reduced, which increases the pinning strength, i.e., concentrat ion and probable size of the vacancy clusters. The peak that appears f irst very close below the melting line broadens, increases in height, and shifts to lower fields going from the overdoped into the optimally doped region. Above the peak field B, the current becomes less sensit ive to the growing strength of the defect structure in accordance with a plastic deformation of vortices. In the field region below B, the v ery low current density is related to a collective interaction. The tr ansition of this elastic interaction below B, into a regime of plastic deformation above, initiated by the thermal softening of the shear mo dulus, results in the rise of the current density. The shift of this t ransition to lower magnetic fields with increasing oxygen reduction as well as with decreasing temperature is related to the thermal influen ce on the distribution of pinning energies that result in a temperatur e-dependent effective concentration of pinning defects.