Phase transitions in a two-dimensional vortex lattice with defects: Monte Carlo simulation

Abstract

Phase diagrams and phase transitions in a two-dimensional vortex lattice with defects have been investigated by Monte Carlo (MC) simulation techniques. It was found that in the presence of defects, the melting of the vortex lattice proceeds in two stages. First, the ideal triangular lattice transforms at low temperature into islands, which are pinned to the defects and rotate around them (“rotating lattice” phase). Then, at a higher temperature, the boundaries of the “vortex lattice islands” become smeared and the system transforms into a vortex liquid. The dependencies of phase transition temperatures on pinning potential have been obtained. The visual pictures of the flux lattice structures have been calculated. The current–voltage characteristics (IVC) of the vortex structures with defects in forms of points, lines and squares were simulated at different temperatures and defects concentration. We have found that the point defects are more efficient than the other type of defects with equivalent specific concentration. It was shown that the dependencies of critical current on temperature and defects' concentrations are directly associated with the phase state of the vortex system.