Monolayer Solid Helium-4 Clusters on Graphite

TL;DR

This study uses path integral Monte Carlo simulations to demonstrate that low-density monolayer helium-4 on graphite forms solid clusters and vapor, challenging previous claims of superfluidity and explaining heat capacity features.

Contribution

It provides direct computational evidence that the low-density phase consists of solid clusters and vapor, clarifying the phase behavior of helium-4 on graphite.

Findings

Low-density monolayer has solid clusters and vapor.

Heat capacity peaks are due to melting of solid clusters.

The system is not in a superfluid phase at low densities.

Abstract

In order to resolve the controversy about the low density region of the phase diagram of the 4He monolayer on graphite, we have undertaken a path integral Monte Carlo study of the system. We provide direct evidence that the low density monolayer possesses solid clusters and a low density vapor as opposed to the most recent proposal that the system is in a superfluid phase. We further establish that the rounded heat capacity peaks observed at low densities are caused by melting of such solid clusters and are not associated with the suggested superfluid transition.