FIELD-INDUCED SUPPRESSION OF THE PHASE-TRANSITION IN BI2SR2CACU2O8

Magnetization measurements in field on a Bi2Sr2CaCu2O8 single crystal do not reveal any sharp phase transition between the normal and superconducting states. The M(H) curves for temperatures about T(c) in fields up to 5 T all display diamagnetic behavior. Above the mean-field transition temperature T(c)MF, quasi-two-dimensional diamagnetic fluctuations are responsible. Below T(c)MF strong deviations from the conventional mean-field (Abrikosov) behavior lead to an apparent T(c)(B) which increases with field. This effect clearly demonstrates the inadequacy of the mean-field theory to describe the vortex state below the crossover line H(c2)(T).