Static and Dynamic Coupling Transitions of Vortex Lattices in Disordered Anisotropic Superconductors

Abstract

We use three-dimensional molecular dynamics simulations of magnetically interacting pancake vortices to study vortex matter in disordered, highly anisotropic materials such as BSCCO. We observe a sharp 3D-2D transition from vortex lines to decoupled pancakes as a function of relative interlayer coupling strength, with an accompanying large increase in the critical current remniscent of a second peak effect. We find that decoupled pancakes, when driven, simultaneously recouple and order into a crystalline-like state at high drives. We construct a dynamic phase diagram and show that the dynamic recoupling transition is associated with a double peak in dV /dI. PACS numbers: 74.60Ge, 74.60.JgIn highly anisotropic superconductors such as BSCCO, the vortex lattice is composed of individual pancake vortices [1]. These pancakes, which interact both magnetically and through Josephson coupling, align under certain conditions into elastic lines resembling those found in isotropic superconductors. Three-dimensional (3D) linelike behavior has been observed in transformer geometry measurements [2], muon-spin-rotation [3] and neutron scattering [4]. Under different conditions, however, the pancake vortices in each layer move independently of the other layers, and the system acts like a stack of independent thin film superconductors. Such two-dimensional (2D) behavior has also been seen in transformer experiments [5]. Thus in layered superconductors two different effective dimensionalities of the vortex pancake lattice may appear, each with different characteristic properties.Layered superconductors exhibit a striking second peak in magnetization measurements [4,[6][7][8][9][10], corresponding to an abrupt increase in the critical current of the material as the applied field is increased. This second peak is especially sharp in BSCCO, as shown in local Hall probe measurements [7,9,10] and recent Josephson plasma frequency measurements [11]. The lattice appears ordered at fields below the transition and disordered above. There seems to be no widely accepted agreement on the mechanism behind this transition, although numerous scenarios have been suggested, including vortex entanglement [13], dislocation proliferations [12], dynamic effects [14], or a 3D to 2D transition in the vortex pancake lattice [4,7,15,16,18]. The effect of strong disorder on a possible 3D-2D transition is unclear, and also it is not known how the transport properties would be affected by a 3D-2D transition.In 2D systems with uncorrelated pinning, the vortex lattice can be dynamically reordered by an applied driving current, passing from a glassy state at zero drive, through plastic flow [19], to a reordered state at high current [20][21][22][23]. In layered systems, when the between plane interactions of pancakes is weak enough that the pancakes are decoupled, the pancakes on each plane should reorder when a high enough driving current is applied. This reordering may also be accompanied by a dynamically driven recoupling transition [20], but it is unclear w...

Metastability and transient effects in vortex matter near a decoupling transition

“…As a function of applied field, temperature, or interlayer coupling the model exhibits a sharp 3D (coupled, ordered phase) to 2D (decoupled, disordered phase) transition, consistent with theoretical expectations [12,13], that is associated with a large change in the critical current [30]. Near the disordering transition, we find strong metastability and transient effects.…”

Do Vortices Entangle?

“…If this pancake decouples from the rest of the vortex line below, then the angular force as a function of angular displacement will not follow the form shown in Fig 2. but will instead be periodic in the winding angle, due to the interaction with the remaining stack of pancake vortices left behind. This is illustrated by a simulation of a pancake vortex system [23] shown in Fig. 3.…”

Effects of Point Defects on the Phase Diagram of Vortex States in High-TcSuperconductors in theB‖Nonomura1, Hu2 2001Phys. Rev. Lett.787741View full textAdd to dashboardCite

“…Numerically, difference between the BG and VG phases was discussed, [14][15][16] and the overall phase diagram was obtained recently. [17,18] However, studies based on thermodynamic quantities are still lacking.…”