Localization versus inhomogeneous superfluidity: Submonolayer He 4 on fluorographene, hexagonal boron nitride, and graphene

International audienceWe study a submonolayer 4He adsorbed on fluorographene (GF) and on hexagonal boron nitride (hBN) at low coverage. The adsorption potentials have been computed ab initio with a suitable density functional theory including dispersion forces. The properties of the adsorbed 4He atoms have been computed at finite temperature with path integral Monte Carlo and at T=0K with variational path integral. From both methods we find that the lowest energy state of 4He on GF is a superfluid. Due to the very large corrugation of the adsorption potential this superfluid has a very strong spatial anisotropy, the ratio between the largest and smallest areal density being about 6, the superfluid fraction at the lowest T is about 55%, and the temperature of the transition to the normal state is in the range 0.5–1 K. Thus, GF offers a platform for studying the properties of a strongly interacting highly anisotropic bosonic superfluid. At a larger coverage 4He has a transition to an ordered commensurate state with occupation of 1/6 of the adsorption sites. This phase is stable up to a transition temperature located between 0.5 and 1 K. The system has a triangular order similar to that of 4He on graphite but each 4He atom is not confined to a single adsorption site and the atom visits also the nearest neighboring sites giving rise to a novel three-lobed density distribution. The lowest energy state of 4He on hBN is an ordered commensurate state with occupation of 1/3 of the adsorption sites and triangular symmetry. A disordered state is present at lower coverage as a metastable state. In the presence of an electric field the corrugation of the adsorption potential is slightly increased but up to a magnitude of 1 V/Å the effect is small and does not change the stability of the phases of 4He on GF and hBN. We have verified that also in the case of graphene such electric field does not modify the stability of the commensurate √3×√3R30∘ phase