# Prediction for two spatially modulated superfluids: $^4$He on fluorographene and on hexagonal BN

## TL;DR

This study predicts that monolayer $^4$He adsorbed on fluorographene and hexagonal boron nitride is a superfluid with significant spatial density modulation, based on ab initio potentials and quantum simulations.

## Contribution

The paper introduces new ab initio adsorption potentials for $^4$He on GF and hBN and predicts a superfluid phase with strong spatial modulation, differing from behavior on graphite and graphene.

## Key findings

- $^4$He on GF and hBN is a fluid, not localized.
- The superfluid state exhibits large density modulation.
- Predicted superfluid has stronger spatial modulation than previously observed.

## Abstract

We have derived the adsorption potential of $^4$He atoms on fluorographene (GF), on graphane and on hexagonal boron nitride (hBN) by a recently developed ab initio method that incorporates the van der Waals interaction. The $^4$He monolayer on GF and on hBN is studied by state-of-the-art quantum simulations at T=0 K. With our adsorption potentials we find that in both cases the ground state of $^4$He monolayer is a fluid and not an ordered state with localized atoms as on graphite and on graphene. In the case of GF the present result is in qualitative agreement with the superfluid phase that was obtained using an empirical adsorption potential [M. Nava et al., Phys. Rev. B 86, 174509 (2012)]. This fluid state of $^4$He on GF and on hBN is characterized by a very large density modulation and at the equilibrium density the ratio $\Gamma$ between the largest and the smallest local density along the direction of two neighboring adsorption sites and averaged over the perpendicular direction is $\Gamma$ = 1.91 for GF and $\Gamma$ = 1.65 for hBN. Recent experiments [J. Nyeki et al., Nature Physics 13, 455 (2017)] have discovered a superfluid phase in the second layer $^4$He. This is a spatially modulated superfluid that turns out to have anomalous thermal properties. This gives a strong motivation for an experimental study of monolayer $^4$He on GF and on hBN that we predict to be a superfluid with a much stronger spatial modulation.