Low-density phases of He 3 monolayers adsorbed on graphite

Quantum Monte Carlo simulations at zero temperature of a He3 monolayer adsorbed on graphite, either clean or preplated with He4, unexpectedly point to a gas-liquid phase transition at a very low areal density of the order of 0.01Å-2. This result stems from an essentially unbiased calculation of the ground-state energy for an infinite, defect-free substrate, which interacts with He atoms via a realistic potential, whereas the interaction between two He atoms includes two- and three-body terms. The sensitivity of the gas-liquid coexistence region on the model Hamiltonian employed is discussed.