Nonequilibrium Vortex Melting inBi2Sr2CaCu2O8+δCrystals

We present evidence for nonequilibrium two stage melting in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$. Samples cooled in low applied magnetic fields display a sharp hysteretic drop of the resistance which is associated with a first-order vortex lattice freezing transition. Using an eight-terminal geometry, we have measured a second sharp feature in the liquid phase which suggests thermal interlayer decoupling of vortex lines. The melting transition temperature decreases with increasing current, however, indicating that these measurements were made under nonequilibrium conditions.