Effect of Quenched Disorder in the Entropy-Jump at the First-Order Vortex Phase Transition of Bi2Sr2CaCu2O8+δ

We study the effect of quenched disorder in the thermodynamic mag- nitudes entailed in the first-order vortex phase transition of the extremely layered Bi2Sr2CaCu2O8+δ compound. We track the temperature-evolution of the enthalpy and the entropy jump at the vortex solidification transition by means of AC local magnetic measurements. Quenched disorder is introduced to the pristine samples by means of heavy-ion irradiation with Pb and Xe producing a random columnar-track pins distribution with different densities (matching field B� ). In contrast with pre- vious magneto-optical reports, we find that the first-order phase transition persists for samples with Bup to 100Gauss. For very low densities of quenched disorder (pristine samples), the evolution of the thermodynamic properties can be satisfacto- rily explained considering a negligible effect of pinning and only electromagnetic coupling between pancake vortices lying in adjacent CuO planes. This description is not satisfactory on increasing magnitude of quenched disorder.