Abstract
Monolayer films on graphite, remarkably diverse examples of two-dimensional matter, are now well 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 30 commensurate state on graphite is unstable. Results include a superfluid ground state for He and a fluid ground state for He, 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 He 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 He 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.
10 More- Received 24 April 2012
DOI:https://doi.org/10.1103/PhysRevB.86.174509
©2012 American Physical Society