First-order disorder-driven transition and inverse melting of the vortex lattice

Vortex matter phase transitions in the high-temperature superconductor Bi2Sr2CaCu2O8 were studied using local magnetization measurements combined with a vortex 'shaking' technique. The measurements revealed thermodynamic evidence of a first-order transition (FOT) along the second magnetization peak line. at temperatures below the apparent critical point T-cp. We found that the FOT line does not terminate at T-cp, but continues down to at least 30 K. This observation suggests that the ordered vortex lattice phase is destroyed through a unified FOT that changes its character from thermally induced melting at high temperatures to a disorder-induced transition at low temperatures. At intermediate temperatures the transition fine shows an upturn. which implies that the vortex matter displays 'inverse' melting behavior. (C) 2001 Published by Elsevier Science B.V.