Peak effect of critical current in an untwinned Y-123 superconducting single crystal

Broad double peaks of the critical current density were observed in an untwinned Y-123 single crystal prepared by a pulling technique. Force-displacement characteristics of flux lines were measured using the Campbell method in the vicinity of the peak fields. It was found that the interaction distance decreased monotonically in a wide range of the field, while the increasing rate of the Labusch parameter changed appreciably at the magnetic field at which the critical current density started to increase. These results are completely different from those in the case of Bi-2212. This fact seems to suggest that the peak effect is ascribed to the change in the flux-pinning mechanism but not to the change in the property of flux lines. Field-induced pinning by weakly superconducting regions seems to be a candidate for the responsible pinning mechanism. However, the characteristic field at which the critical current density starts to increase increases with elevating temperature. Hence, this temperature dependence cannot be explained by such a simple mechanism. The synchronization mechanism known in metallic superconductors may also work in Y-123.