Disordering transitions in vortex matter: peak effect and phase diagram

Abstract

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. PACS numbers: 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 J c and magnetization. In highly anisotropic superconductors, such as BSCCO, a pronounced fish-tail or peak effect, in which J c shows a sharp increase, is observed as a function of increasing field [2]. 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 [3]. The peak has also been proposed to arise from plasticity, proliferation of in-plane defects, dynamical effects, or matching effects [4][5][6].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 [7] and muon lifetime [8] 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 [9] and magneto-optical studies [10] point to the first-order nature of this transition. In YBCO a rapid increase in J c as a function of magnetic field is observed and is thought to indicate a transition from an ordered to a disordered state. History and memory effects found near this peak indicate that this transition is also first order [5], suggesting that the physics of the second peak is similar in BSCCO and YBCO. In recent muon-spin measurements in YBCO, the order-disorder transition has been interpreted to be a 3D-2D transition of the vortex lattice [11]. As a function of temperature, in YBCO a peak effect can be observed very near or at the melting line [12,13]. Transformer measurements in this regime provide evidence of vortex cutting in the liquid state, suggesting that the breakdown of the 3D nature of vortex lines is relevant in this case as well.A key question is what type of sharp transition is responsible for the observed peak in J c , and whether the mechanism of the peak effect is the same going through the melting line as through the second peak. In this paper we argue that it is a decoupling transition alon...

Phase diagram of vortex matter of type-II superconductors

“…3. In fact, the reentrance of vortex melting has been predicted by theory and confirmed by experiments [5][6][7][8][9] . We then study the vortex phases as functions of field B and κ (and/or q).…”

Phase behavior of type-II superconductors with quenched point pinning disorder: A phenomenological proposal

“…Recent simulation work on a pancake vortex model [85] indicates that the PE may be associated primarily with a decoupling transition in the c-axis direction, in which c-axis correlations of vortices become short-ranged. This author and collaborators have argued recently that modelling the MG phase in a pancake vortex model in terms of a random stacking of perfectly crystalline layers should yield a fairly high-energy situation, with an energy which scales linearly with the area of each plane [21].…”

Decoupling and depinning of flux lattices in disordered layered superconductors

“…Melting, e.g., is expected to occur by thermal or disorder induced dislocations, as indeed demonstrated for fields parallel to the layers 25,26 . Numerical simulations on related XY models have also shown disorder induced melting 27,28,29 . The flux lattice can undergo a transition which is unique to layered superconductors, i.e.…”

“…while σ (2) αβ and B (2) αβ have the previous forms (25,29). In the hierarchical scheme G −1 is represented by [ã, a(u)] which are now given byã…”