Unified order-disorder vortex phase transition in high-T_c superconductors

The diversity of vortex melting and solid-solid transition lines measured in different high-T_c superconductors is explained, postulating a unified order-disorder phase transition driven by both thermally- and disorder-induced fluctuations. The temperature dependence of the transition line and the nature of the disordered phase (solid, liquid, or pinned liquid) are determined by the relative contributions of these fluctuations and by the pinning mechanism. By varying the pinning mechanism and the pinning strength one obtains a spectrum of monotonic and nonmonotonic transition lines similar to those measured in Bi₂Sr₂CaCu₂O₈, YBa₂Cu₃O_7-δ, Nd_1.85Ce_0.15CuO_4-δ, Bi_1.6Pb_0.4Sr₂CaCu₂ O_8+δ, and (La_0.937Sr_0.063)₂CuO₄.