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 line shows an upturn, which implies that the vortex matter displays 'inverse' melting behavior.