Vortex-matter phase transitions in Bi2Sr2CaCu2O8: Effects of weak disorder

The vortex matter phase diagram in Bi2Sr2CaCu2O8 crystals is investigated by introducing very low doses of point and correlated disorder. We conclude that at least three distinct phases are present. The ordered low-field quasilattice phase has a finite shear modulus which vanishes at the first-order melting or sublimation transition at elevated temperatures. At lower temperatures the quasilattice transforms into a disordered solid as field is increased above the second magnetization peak. This disorder-driven transition shifts to lower fields with increased point disorder. The first-order transition displays corresponding downward curvature in the vicinity of the critical point.