Vortex matter in Bi₂Sr₂CaCu₂O₈ with pointlike disorder

We investigate the effect of point-like disorder, introduced by irradiation with 2.3 MeV electrons, on the mixed state phase diagram of Bi ₂Sr₂CaCu₂O₈ single crystals. We focus on the higher irradiation doses that produce a significant depression of the critical temperature T_c, to as low as 2/3 of the initial value. Surprisingly, the first order phase transition (FOT) of the vortex ensemble, from a crystal to the pancake vortex liquid, persists in those highly disordered samples. The second peak in the irreversible magnetization, observed at low temperatures, is equally observed after high irradiation doses, but at much lower magnetic fields. A simple scaling of the phase diagram for samples with various degrees of disorder is not possible, indicating that several fundamental parameters of the superconductor are affected. From the analysis of the angular dependence of the FOT, we deduce that the effective anisotropy factor increases after irradiation.