Fluctuation-Induced First-Order Phase Transition in an Anisotropic Planar Model ofN2on Graphite

Monte Carlo simulations are used to show that the orientational ordering transition of planar quadrupoles on a triangular lattice is first order. The orientational ground state is a herringbone structure, and the transition lies in the universality class of a ($n=3\mathrm{}$, $d=2\mathrm{}$) Heisenberg model with face-oriented cubic anisotropy. The Monte Carlo results are related to recent low-energy electron-diffraction studies of ${\mathrm{N}}_2$ on graphite. Domainwall motion and the applicability of this model to herringbone phases of liquid crystals are also discussed.