Effective interaction between He atoms on a graphite surface

Evidence is presented to show that the two-dimensional effective interaction between He atoms on graphite is less attractive than the free-space interaction Vo. The principal origin of the effect is substrate screening of the attraction. A small shift arises also from averaging the adatom motion perpendicular to the surface. The revised potential Veff is shown to yield improved agreement for the specific heat in the low-coverage region where a quantum virial expansion applies.