Phase Diagram of Bi 2 Sr 2 CaCu 208 in the mixed state : effects of anisotropy and disorder

The vortex-lattice phase transitions in Bi2Sr2CaCu208 crystals with various oxygen stoichiometry and artificially induced disorder are studied using local magnetization measurements. The first-order phase transition line at elevated temperatures shifts upward for more isotropic over-doped samples. At lower temperatures another sharp transition is observed at the onset of second magnetization peak. The two lines merge at a multicritical point at intermediate temperatures forming apparently a continuous phase transition line that is anisotropy dependent. Weak point disorder preserves the first-order transition and shifts it to lower fields. This shift is accompanied by a corresponding downward shift of the second magnetization peak. A low density of columnar defects shifts the first-order transition to higher fields, apparently transforming it into a continuous transition. The first-order transition in clean cry.~tals is concluded to involve melting of the vortex-lattice. Schematic phase diagram based on this study is presented.