Computer Simulation Studies of Flux Lines in a Model Layered Superconductor

This work studies the physics of fluctuating flux lines in the mixed state of a model layered superconductor. We have applied a computer simulation technique to address some of the crucial questions at a phenomenological level. Starting from the Lawrence-Doniach model for layered superconductors, we have derived approximate pairwise interactions for the vortices and applied the Monte Carlo technique taking vortex core positions as basic physical variables. For a clean system, we observe that the flux lattice melts in the large region of the B-T phase diagram and find that the Lindemann criterion number is a field B dependent quantity. By performing the Delaunay triangulation on vortex configurations, we analyze the underlying physics in terms of topological defects. An important aspect of the clean flux lines system is the nature of the dimensional crossover as the vortex density is varied. In the low field solid phase, topological defects penetrate the system at the melting temperature as pairs of oppositely charged long line defects while in the high field, they appear as point-like objects. In the melted liquid phase, an individual line goes through a distinct dimensional crossover. This is understood in terms of an onset of cutting on the smallest length scale which defines the decoupling between the adjacent superconducting layers. This is distinct from disappearance of the long range phase coherence which should have occurred at or in the close vicinity of the melting transition. When strong pins are present, we observe that the simulated I-V characteristics under an applied tilt potential follow a basic vortex glass-like scaling relation in the vicinity of the depinning temperature. It was also found that strong anisotropy and presence of random point pins may drive the system from a 3D quasi-lattice phase to a disorder dominated glass state as the the vortex density increases at low temperatures. We interpret this glass transition in terms of 2d-topological defects penetrating the sample.