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

Phase diagrams and phase transitions in a two-dimensional vortex lattice wi th defects have been investigated by Monte Carlo (MC) simulation techniques . It was found that in the presence of defects, the melting of the vortex l attice proceeds in two stages. First, the ideal triangular lattice transfor ms at low temperature into islands, which are pinned to the defects and rot ate around them (''rotating lattice'' phase). Then, at a higher temperature , the boundaries of the "vortex lattice islands" become smeared and the sys tem transforms into a vortex liquid. The dependencies of phase transition t emperatures on pinning potential have been obtained. The visual pictures of the flux lattice structures have been calculated. The current-voltage char acteristics (IVC) of the vortex structures with defects in forms of points, lines and squares were simulated at different temperatures and defects con centration. We have found that the point defects are more efficient than th e other type of defects with equivalent specific concentration. It was show n that the dependencies of critical current on temperature and defects' con centrations are directly associated with the phase state of the vortex syst em. (C) 2000 Elsevier Science B.V. All rights reserved.