High Pressure Hydrogen and the Potts Model on a Triangular Lattice

We present Monte Carlo studies and analysis of the frustrated
antiferromagnetic Potts model of a triangular lattice. This Potts model shows a
remarkably rich range of structures, and striking similarities to the high pressure
phases of hydrogen which are typified by hexagonal close packed layered structures[1]. There are four known H2 molecular phases, all of which are isostructural to within the resolution of X-ray diffraction. Experimentally, the phase lines have been mapped by spectroscopy, which cannot reveal the structure. Study by density functional theory (DFT) has suggested a large number of candidate structures, based on the hexagonalclose packing of H2 molecules. The Potts model exhibits structures similar to DFT candidate hydrogen phases I, II and III: the range of different Potts model structures suggests that the hydrogen system in the "Phase II" region, may exhibit more than a single phase. It also suggests reorientational excitations which may be detectable in spectroscopy.