VORTEX MATTER PHASE-TRANSITIONS IN BI2SR2CACU2O8

Vortex-lattice phase transitions in Bi2Sr2CaCu2O8 crystals are studied using local magnetization measurements. The vortex matter is found to exhibit at least three distinct phases. A rather ordered quasilattice phase is present at low fields. At elevated temperatures the quasilat tice melts (or sublimates) through a first-order phase transition, whe reas at lower temperatures a transition to a highly disordered vortex solid occurs at the second magnetization peak. A very low density of c olumnar defects results in significant pinning of the quasilattice, de monstrating the finite shear modulus of this phase. Local ac measureme nts reveal that in the presence of columns the pinned quasilattice sti ll melts through a first-order phase transition with an observable ste p in equilibrium magnetization at elevated temperatures.