Peak effect in type II superconductors

Results of investigations of the behavior of the static (I/sub c/) and dynamic (I/sub d/) critical currents near H/sub c/(T) are reported, where the former is determined for constant threshold voltage, and the latter by an extrapolation of the linear part of the current--voltage characteristic (CVC) to zero voltage. The measurements were performed for a series of specimens (kappa=50) with different microstructures in a broad temperature range. A peak effect is detected on I/sub d/(H) vertical-bar/sub T/ and I/sub c/(H) vertical-bar/sub T/. In certain cases, two peaks, which differ from each other in their temperature dependence, are observed on I/sub c/(H), where the high field peak is similar in its behavior to the dynamic current peak. The presence of the lower peak, which is accompanied by a large modification in the lower part of the CVC is explained by an increase in the degree of synchronization of vortex interaction with pinning centers in the static state of the vortex latice as H..-->..H/sub c/(T). The peak of the dynamic critical current is related to an increase in the number and (or) efficiency of the pinning centers near the upper critical magnetic field, where the strong temperature dependence of the peak magnitudemore » obtained (approx.(H/sub c/(T))/sup 4.6/ can be explained under an assumption about the importance of the influence of inhomogeneities in the electron mean free path.« less