Vortex-matter phase transitions in ${\mathbf{Bi}}_2{\mathbf{Sr}}_2{\mathbf{CaCu}}_2{\mathbf{O}}_8:$ Effects of weak disorder

The vortex matter phase diagram in Bi${}_2$Sr${}_2$CaCu${}_2$O${}_8$ 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.