On the Phase Diagrams of 4He Adsorbed on Graphene and Graphite from Quantum Simulation Methods

The ground-state phase diagrams of He-4 adsorbed on graphene and graphite are calculated using quantum simulation methods. In this work, a systematic investigation of the approximations used in such simulations is carried out. Particular focus is placed on the helium-helium (He-He) and helium-carbon (He-C) interactions, as well as their modern approximations. On careful consideration of other approximations and convergence, the simulations are otherwise (numerically) exact. The He-He interaction as approximated by a sum of pairwise potentials is quantitatively assessed. A similar analysis is made for the He-C interaction, but more thoroughly and with a focus on surface corrugation. The importance of many-body effects is discussed. Altogether, the results provide "reference data" for the considered systems. Using comparisons with experiments and first-principle calculations, conclusions are drawn regarding the quantitative accuracy of these modern approximations to these interactions.