Dynamics, topological defects and phase transitions in ordered media

Abstract

In many different physical systems, the symmetry breaking due the transition to an ordered phase allows for the existence of stable topological defects that have a dominant role in determining their physical properties. Such defects arise as a consequence of the periodicity and stiffness of the ordered media and are responsible for the mechanisms which lead to the phase transition. In atomic crystal lattices, for example, topological defects in the form of dislocations determine most of the plastic properties of the solids, both in the dynamics and statics. High temperature superconductors are another example of ordered media where the dynamics of topological defects in the form of vortices is predominant, being responsible for the resistivity of the superconducting material. Other important examples of ordered media where topological defects are dominant occur in crystal surfaces such as in strained epitaxial layers and boundary lubrication. In this case, the ordered media consists of one or several superposed atomic layers under the action of an external force induced by the substrate or by the other surface. Studies of the dynamics of defect nucleation, such as in the case of strained epitaxial layers in semiconductors, allows for a better understanding of the ordering mechanism and dislocation formation and may eventually help in the fabrication of structures and devices. In view of the possible applications of these structures in nanotechnology, it is of fundamental importance to advance the knowledge in this field. Likewise, studies of siding friction of lubricated surfaces are of great importance for nanotribology. The goal of the project is to develop theoretical research on the dynamics, topological defects and phase transitions in ordered media within a unified approach. The problems to be investigated and the analytical and numerical methods used are common to the different fields. Numerical methods based on Monte Carlo, molecular-dynamics and Langevin-dynamics simulations will be used in the studies of the relevant models. Important computational limitations like long execution times, due to the slow relaxation and of the systems, should be overcome by the use of parallel processing. Among the problems to be investigated are: vortex lattice and quantum fluctuations in superconductors, sliding friction on surfaces and dislocation nucleation in strained epitaxial layers. (AU)