Multiple changes of order of the vortex melting transition in Bi2Sr2CaCu2O8 with dilute columnar defects.

A low concentration of columnar defects is reported to transform a first-order vortex lattice melting line in Bi2Sr2CaCu2O8 crystals into alternating segments of first- and second-order transitions separated by two critical points. As the density of columnar defects is increased, the critical points shift apart and the range of the intermediate second-order transition expands. The measurement of equilibrium magnetization and the mapping of the melting line down to 27 K was made possible by employment of the shaking technique.

Interplay of anisotropy and disorder in the doping-dependent melting and glass transitions of vortices in Bi2Sr2CaCu2O 8+delta.

We study the oxygen doping dependence of the equilibrium first-order melting and second-order glass transitions of vortices in Bi2Sr2CaCu2O 8+delta. Doping affects both anisotropy and disorder. Anisotropy scaling is shown to collapse the melting lines only where thermal fluctuations are dominant.