Low field irreversibility in twinned and untwinned YBaCuO crystals

An abrupt collapse of the irreversibility line is observed in both twinned and untwinned YBaCuO crystals within a critical regime. The exponent α of the power law describing the irreversibility line Hirr changes from 4 3 at high fields to larger values (between 1.5 and 2) below the collapse. We propose that the observed low field anomaly, which extends down to temperatures well below the irreversibility line, is associated with the thermal softening boundary which increases as T2 and crosses Hirr shifting it to lower fields. Below this crossover, which occurs when the mean-square thermal displacement √ 〈 u2 〉 of the vortices exceeds coherence length ξ, the strength of the pinning is strongly reduced. © 1993.