VORTEX PHASE-DIAGRAM OF BI2SR2CACU2O8-Y NEAR THE SUPERCONDUCTING TRANSITION

Six-terminal measurements of the resistive transition of Bi2Sr2CaCu2O8 -y crystals in a weak external magnetic field H parallel-to c axis are analyzed to deduce the magnetic H-T phase diagram near the supercondu cting transition. We find evidence for two magnetic field boundaries, namely, a depinning field H(p)(T), and a 3D-2D crossover field H(m)(T) greater-than-or-equal-to H(p)(T), each of which intersects the H = 0 axis at a temperature above the zero-field a-b plane transition temper ature. We interpret the crossover field H(m)(T) as marking the boundar y between 3D vortex lines and interacting 2D vortices.