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Ground state of monolayer 4He/graphite: Quantum liquid versus quantum solid.

Two extensions are made to calculations which represent monolayer 4 He/graphite as a two-dimensional (2D) version of 3D helium: inclusion of substrate-mediated interactions and of modulation of the liquid phase by the substrate corrugation potential. A variation-perturbation approximation is developed for the modulation energy. The McLachlan energy is a substantial correction to the 2D energy calculation, while the energy of elastic distortion of the substrate is a small term. Consequences for the nature of the ground state of 4 He/graphite are discussed. If the McLachlan energy is a for approximation to the energy shift by absorption-induced processes and the substrate corrugation has, or is somewhat less than, the value used in recent work, the calculated ground state is a quantum liquid