Phase diagram of $^{4}$He on graphene

TL;DR

This study uses computer simulations to map the phase diagram of helium-4 on graphene, revealing solid, superfluid, and incommensurate phases, with results similar to helium on graphite.

Contribution

First detailed simulation-based phase diagram of helium-4 on graphene, identifying multiple phases and layer promotion coverage.

Findings

Two commensurate solid phases at 1/3 and 7/16 fillings

Superfluid phase observed in the second layer

No superfluid phase in the first layer

Abstract

The low temperature phase diagram of $^4$He adsorbed on a single graphene sheet is studied by computer simulation of a system comprising nearly thousand helium atoms. In the first layer, two commensurate solid phases are observed, with fillings 1/3 and 7/16 respectively, separated by a domain wall phase, as well as an incommensurate crystal at higher coverage. No evidence of a thermodynamically stable superfliuid phase is found for the first adlayer. Second layer promotion occurs at a coverage of 0.111(4) $\AA^{-2}$. In the second layer two phases are observed, namely a superfluid and an incommensurate solid, with no commensurate solid intervening between these two phases. The computed phase diagram closely resembles that predicted for helium on graphite.