Peak effect and its evolution from oxygen deficiency in YBa2Cu3O7-δ single crystals

The current density j in bulk YBa2Cu3O7-delta frequently shows a maximum at fields far above the self-field. The responsible defect structure for this peak effect (PE) an small clusters of oxygen vacancies, impurity atoms, or dopants. The current density caused by these defects is studied during the evolution of the PE. Very pure, twin-free crystals without a j(B) peak after high-pressure oxidation were subsequently oxygen reduced, which increases the pinning strength, i.e., concentration and probable size of the vacancy clusters. The peak that appears first 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 sensitive to the growing strength of the defect structure in accordance with a plastic deformation of vortices. In the field region below B, the very low current density is related to a collective interaction. The transition of this elastic interaction below B, into a regime of plastic deformation above, initiated by the thermal softening of the shear modulus, results in the rise of the current density. The shift of this transition to lower magnetic fields with increasing oxygen reduction as well as with decreasing temperature is related to the thermal influence on the distribution of pinning energies that result in a temperature-dependent effective concentration of pinning defects.