Decoupling and depinning of flux lattices in disordered 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 disorder generates a random Josephson coupling between layers which leads to a Josephson glass (JG) phase at low temperatures; vanishing of the JG order identifies a depinning transition. We also find that disorder and thermal fluctuations lead to layer decoupling where the renormalized Josephson coupling vanishes. Near decoupling an anharmonic regime is found, where usual elasticity and the resulting Bragg glass are not valid. The depinning line crosses the decoupling line at a multicritical point, resulting in four transition lines. The phase diagram is consistent with the unusual data on Bi₂Sr₂CaCu₂O₈ such as the “second peak” and depinning transitions. The Josephson plasma frequency is evaluated in the various phases.