Disordering transitions in vortex matter: peak effect and phase diagram

Using numerical simulations of magnetically interacting vortices in disordered layered superconductors we obtain the static vortex phase diagram as a function of magnetic field and temperature. For increasing field or temperature, we find a transition from ordered straight vortices to disordered decoupled vortices. This transition is associated with a peak effect in the critical current. For samples with increasing disorder strength the field at which the decoupling occurs decreases. Long range, nonlinear interactions in the c-axis are required to observe the effect. 2002 Published by Elsevier Science B.V. PACS: 74.60.Ge; 74.60.Jg Vortex matter in superconductors exhibits a remarkably rich variety of distinct phases due to the numerous competing interactions [1]. These phases can strongly affect the response of the system, such as the critical current Jc and magnetization. In highly anisotropic superconductors, such as BSCCO, a pronounced fish-tail or peak effect, in which Jc shows a sharp increase, is observed as a function of increasing field [2–8]. This peak can be interpreted as occurring when the increasing field weakens the interlayer coupling of vortex pancakes due to geometric constraints, and a transition occurs from weakly pinned 3D line vortices to decoupled 2D pancake vortices which can more easily adjust their positions to maximize the pinning [9–11]. The peak has also been proposed to arise from plasticity, proliferation of in-plane defects, dynamical effects, or matching effects [12–17]. There is mounting experimental evidence that the peak effect is associated with a sharp transition in the vortex lattice from an ordered state to a disordered state. In BSCCO, neutron scattering [18] and muon lifetime [19] experiments provide evidence that a transition from an ordered 3D vortex arrangement to a disordered or decoupled arrangement is associated with the peak effect. Additional evidence from plasma-resonance [20] Corresponding author. Tel.: +1-505-665-1134; fax: +1-505-