Josephson glass and decoupling of flux lattices in layered superconductors

Phase transitions of a flux lattice in layered superconductors with magnetic field perpendicular to the layers and in presence of disorder are studied. We find that the Josephson coupling between layers leads to a strongly pinned Josephson glass (JG) phase at low temperatures and fields. The JG phase undergoes either a decoupling transition with increasing field or a depinning transition with increasing temperature. The resulting phases undergo further depinning and decoupling transitions, respectively, resulting in a phase diagram with a multicritical point where four phases meet. The phase diagram accounts for unusual data on Bi_2Sr_2CaCu_2O_8 such as the "second peak" transition and the recently observed depinning transitions.