Stability of He4 superlattices on graphite

Using the variational method proposed by Lowy and Woo, we calculate the ground-state energies of the 13- and 1/4-coverage superlattices of He4 physisorbed on graphite. The properly symmetrized single-particle factors in the trial wave function are constructed from expansions in an orthogonal basis set whose elements are the eigenfunctions of a Hamiltonian with a laterally averaged substrate potential. Calculation of the energy of the He4 monolayer in the liquid phase is performed in a manner consistent with the superlattice calculation. The 13-coverage coverage superlattice is found to be stable, while the 1/4-coverage monolayer chooses to remain in the liquid phase.