Novel substrates for Helium adsorption: Graphane and Graphene - Fluoride

The discovery of fullerenes has stimulated extensive exploration of the resulting behavior of adsorbed films. Our study addresses the planar substrates graphene - fluoride (GF) and graphane (GH) in comparison to graphene. We present initial results concerning the potential energy, energy bands and low density behavior of ⁴He and ³He films on such different surfaces. For example, while graphene presents an adsorption potential that is qualitatively similar to that on graphite, GF and GH yield potentials with different symmetry, a number of adsorption sites double that on graphene/graphite and a larger corrugation for the adatom. In the case of GF, the lowest energy band width is similar to that on graphite but the He atom has a significantly larger effective mass and the adsorption energy is about three time that on graphite. Implications concerning the monolayer phase diagram of ⁴He are explored with the exact path integral ground state method. A commensurate ordered state similar to the R30° state on graphite is found the be unstable both on GF and on GH. The ground states of submonolayer ⁴He on both GF and GH are superfluids with a Bose Einstein condensate fraction of about 10%.