PHASE-DIAGRAM OF BI2SR2CACU2O8 IN THE MIXED-STATE - EFFECTS OF ANISOTROPY AND DISORDER

The vortex-lattice phase transitions in Bi2Sr2CaCu2O8 crystals with va rious oxygen stoichiometry and artificially induced disorder are studi ed using local magnetization measurements. The first-order phase trans ition line at elevated temperatures shifts upward for more isotropic o ver-doped samples. At lower temperatures another sharp transition is o bserved at the onset of second magnetization peak. The two lines merge at a multicritical point at intermediate temperatures forming apparen tly a continuous phase transition line that is anisotropy dependent. W eak 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 def ects shifts the first-order transition to higher fields, apparently tr ansforming it into a continuous transition. The first-order transition in clean crystals is concluded to involve melting of the vortex-latti ce. Schematic phase diagram based on this study is presented.