Superconductors, high irreversibility field

The performance of high-temperature superconductors in magnetic fields is intimately related to the upper critical field H (T), the irreversibility line and the critical... 

For strong type-II superconductors a difference may exist between B 2 obtained from ac-susceptibility data and the thermodynamically relevant upper critical field, 5c2, for which (2.13) and (2.14) were derived originally. Prom extensive work on high-Tc cuprates it is known that B 2 is related to the so-called irreversibility line [196]. The basic idea of a semiquantitative flux-creep theory [197] is that pinned vortices can be activated thermally over an energy barrier Uq resulting in a reduced critical current of the form [196]... 

It was previously suggested that positive curvature of the resistively determined Ha T) curve may be a general property of high-Ti cuprate superconductors (Cooper, Loram and Wade 1995, Ovchinnikov and Kresin 1995, 1996, Abrikosov 1997). However, the T-dependence of the resistively determined H 2 T) curve was interpreted as a measure of the T-dependence of the irreversibility line (Almasan et al. 1992a). Indeed, the irreversibility line extracted from zero-field-cooled and field-cooled... 

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