IRREVERSIBILITY LINE OF MONOCRYSTALLINE BI2SR2CACU2O8 - EXPERIMENTAL-EVIDENCE FOR A DIMENSIONAL CROSSOVER OF THE VORTEX ENSEMBLE

The irreversibility line B(m)(T) of monocrystalline superconducting Bi2Sr2CaCu2O8 has been measured in magnetic fields H parallel to the c axis of the crystal. For low inductions B near T(c), we find a parabolic temperature dependence, B(m) almost-equal-to B0(T(c)/T-1)2, while for larger inductions, B(m) grows exponentially with T-1. We argue that the two regimes reflect the three- and the quasi-two-dimensional character of the respective vortex fluctuations. In both regimes, a Lindemann-type melting criterion yields quantitative expressions for B(m)(T) which reproduce the experimental data very well.