Supersolidity in quantum films adsorbed on graphene and graphite

TL;DR

This study uses quantum Monte Carlo simulations to investigate superfluidity in the first layer of helium-4 and hydrogen molecules adsorbed on graphene and graphite, revealing small superfluidity in solid helium-4 and enhancement with vacancies.

Contribution

It provides the first detailed quantum Monte Carlo analysis of superfluidity in adsorbed quantum films on graphene and graphite, highlighting vacancy effects.

Findings

Helium-4 exhibits a small but finite superfluid fraction (0.67%) in the solid phase.

Vacancies increase superfluidity in helium-4 up to 14%.

Hydrogen molecules show no superfluid signal in the perfect solid phase.

Abstract

Using quantum Monte Carlo we have studied the superfluid density of the first layer of $^4$He and H$_2$ adsorbed on graphene and graphite. Our main focus has been on the equilibrium ground state of the system, which corresponds to a registered $\sqrt3 \times \sqrt3$ phase. The perfect solid phase of H$_2$ shows no superfluid signal whereas $^4$He has a finite but small superfluid fraction (0.67%). The introduction of vacancies in the crystal makes the superfluidity increase, showing values as large as 14% in $^4$He without destroying the spatial solid order.