Dodecagonal order in a two-dimensional Lennard-Jones system.

Abstract

We investigate a two-dimensional Lennard-Jones mixture with the interaction parameters chosen so as to favor configurations where the large atoms form squares and equilateral triangles. Many such configurations are possible which by our choice of interactions are nearly degenerate in energy. It is hypothesized that a thermal equilibrium state with 12-fold orientational order exists. Several Monte Carlo simulations were performed to cool the system to a temperature approaching zero. The ordering process was studied by following the evolution of the configurations with temperature. The onset of ordering seemed to be very diffuse in space rather than nucleated at a point. The resulting configurations consist of squares and triangles, except for a few dislocations, and thus have perfect orientational order. We also characterized the deviation from ideal quasiperiodicity in terms of the ``phason strain''; this was analyzed both by fitting a linear relation between the physical space coordinates of the atoms and the corresponding ``perpendicular space'' coordinates, and also by calculating the diffraction peaks. The latter are shifted and broadened, relative to an ideal 12-fold diffraction pattern, as in real quasicrystals.