Supersolidity in the second layer of $para$-H$_2$ adsorbed on graphite

TL;DR

This study uses quantum Monte Carlo simulations to demonstrate that the second layer of para-H2 on graphite can form a supersolid phase with a small but detectable superfluid fraction, revealing new quantum properties of adsorbed hydrogen layers.

Contribution

It provides the first computational evidence of supersolidity in the second layer of para-H2 on graphite, highlighting the role of molecule exchanges in this phase.

Findings

Incommensurate triangular crystal structure observed.

Finite superfluid density detected at specific density.

Superfluid fraction estimated at approximately 0.41%.

Abstract

We calculated the phase diagram of the second layer of $para$-H$_2$ adsorbed on graphite using quantum Monte Carlo methods. The second layer shows an incommensurate triangular crystal structure. By using a symmetric wave function, that makes possible molecule exchanges, we observed that this nearly two-dimensional crystal shows a finite superfluid density around a total density of 0.1650 \AA$^{-2}$. The superfluid fraction of this supersolid phase was found to be small, 0.41$\pm$0.05 \%, but still experimentally accessible.