Adsorption of He isotopes on fluorographene and graphane : fluid and superfluid phases from quantum Monte Carlo calculations

Monolayer films on graphite, remarkably diverse examples of two-dimensionalmatter, are nowwell understood in terms of semiempirical interactions.We explore the phase behavior of helium films on two variants of graphene: graphane (graphene coated with H, denoted GH) and graphene fluoride (GF). The behaviors predicted with quantum Monte Carlo differ qualitatively from those on graphite because of the different surface composition, symmetry, and spacing of the adsorption sites. On both substrates we find that the analog of the standard √3×√3 R30◦ commensurate state on graphite is unstable. Results include a superfluid ground state for 4He and a fluid ground state for 3He, neither of which has been found for the monolayer film on any substrate; these two-dimensional fluids are anisotropic because of the symmetry imposed by the honeycomb lattice of adsorption sites. In the case of 4He on GF the anisotropy is as large as if the superfluid were restricted to move in a multiconnected space, along the bonds of a honeycomb lattice. The superfluid transition temperature at the ground-state density of 4He on GF (GH) is of order 0.25 (1.1) K. At higher coverages both an incommensurate triangular solid and a commensurate state at filling factor 2/7 are found.