Superlattice of parahydrogen physisorbed on graphite surface

We examine the zero-temperature stability of 1/3- and 1/4-coverage superlattices and the liquid phase of parahydrogen physisorbed on graphite by calculating variational ground state energies. A general anisotropic hydrogen-substrate interaction that can be used for both para- and orthohydrogen is first derived by lattice summation. The single-hydrogen Schrödinger equation is solved. Potential parameters are chosen so that the eigenvalue spectrum can be fitted to experimental data. The eigenfunctions are used to form single-particle elements for the many-body wave function. For finite-coverage monolayers of adsorbed hydrogen, the trial wave function is chosen as a product of a properly symmetrized single-particle element and a Feenberg-Jastrow correlating factor. The energies of the liquid and superlattice phases are calculated within the same formalism. We find at 1/3 coverage a stable triangle superlattice. At 1/4 coverage, however, the submonolayer remains in the two-dimensional liquid phase.Work supported in part by National Science Foundation Grant DMR 80-08816.