DOF phase separation of the Lennard-Jones fcc(111) surface

Recent lattice model calculations have suggested that a full-layered crystal surface may undergo, under canonical (particle-conserving) conditions, a preroughening-driven two-dimensional phase separation into two disordered flat (DOF) regions, of opposite order parameter. We have carried out extensive classical molecular dynamics (MD) simulations of the Lennard-Jones fcc(111) surface, to check whether these predictions are relevant or not for a realistic continuous system. Very long simulation times, a grid of temperatures from (2/3)Tm to Tm, and unusually large system sizes are employed to ensure full equilibrium and good statistics. By examining layer-by-layer occupancies, height fluctuations, sublattice order parameter and X-ray structure factors, we find a clear anomaly at ~0.83Tm. The anomaly is distinct from roughening (whose incipiency is also detected at ~0.94Tm), and is seen to be consistent with the preroughening plus phase separation scenario.Comment: REVTeX, 8 pages, 4 figures; new figure showing simulation snapshots added; reference updated and other minor change

Structural and Electronic Properties of a Carbon Nanotorus: Effects of Delocalized Vs Localized Deformations

The bending of a carbon nanotube is studied by considering the structural evolution of a carbon nanotorus from elastic deformation to the onset of the kinks and eventually to the collapse of the walls of the nanotorus. The changes in the electronic properties due to {\it non-local} deformation are contrasted with those due to {\it local} deformation to bring out the subtle issue underlying the reason why there is only a relatively small reduction in the electrical conductance in the former case even at large bending angles while there is a dramatic reduction in the conductance in the latter case at relatively small bending angles.Comment: 10 pages, 6 figure

Thermal attenuation in atom-surface scattering : the two phonon contribution

The specular intensity produced by the scattering of monoenergetic incident particles by a flat surface is calculated as a function of the crystal temperature including one and two virtual phonon processes. The exact procedure developed previously [1, 2] is employed in order to get the matrix elements of the seven different two phonon diagrams. For all the different systems studied (He-Cu, H2-Cu, Ne-Cu) the results show that the most efficient two-phonon term is the diagram of highest order in the perturbation expansion. The total intensity is greater than that obtained by considering the one phonon process alone except at very low crystal temperature. The temperature for which the two phonon contribution becomes non-negligeable is in general low. Comparison with available experimental data gives good agreement in the temperature domain where higher order phonon processes could be neglected.L'intensité du faisceau spéculaire produit par la diffusion d'un faisceau de particules incidentes monoénergétiques par une surface plane, est calculée en fonction de la température du cristal. Le calcul inclut les processus virtuels à un et deux phonons. La procédure exposée précédemment [1, 2] est utilisée et permet de calculer exactement les éléments de matrice des sept différents diagrammes décrivant les échanges virtuels de deux phonons. Pour tous les systèmes étudiés (He-Cu, H2-Cu, Ne-Cu) les résultats indiquent que la contribution la plus importante à l'intensité est donnée par les diagrammes provenant de l'ordre le plus élevé dans le développement en perturbation. L'intensité totale est plus grande que celle donnée par le processus à 1 phonon seul, excepté à très basse température. La température à partir de laquelle la contribution des effets de deux phonons devient appréciable est en général basse. Les intensités calculées reproduisent les données expérimentales dans le domaine de température ou les contributions des processus à 3 phonons ou plus sont négligeables