Vortex Phase Diagram of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8-y}$ near the Superconducting Transition

Six-terminal measurements of the resistive transition of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8-y}$ crystals in a weak external magnetic field $H\parallel c$ axis are analyzed to deduce the magnetic $H-T$ phase diagram near the superconducting transition. We find evidence for two magnetic field boundaries, namely, a depinning field $H_p\mathrm{}\left(T\right)\mathrm{}$ and a 3D-2D crossover field $H_m\mathrm{}\left(T\right)\mathrm{}\ge H_p\mathrm{}\left(T\right)\mathrm{}$, each of which intersects the $H=0\mathrm{}$ axis at a temperature above the zero-field $a-b$ plane transition temperature. We interpret the crossover field $H_m\mathrm{}\left(T\right)\mathrm{}$ as marking the boundary between 3D vortex lines and interacting 2D vortices.