Commensurate-incommensurate transitions in quantum films: Submonolayer molecular hydrogen on graphite

TL;DR

This study uses Path Integral Monte Carlo simulations to explore phase transitions in a monolayer of molecular hydrogen on graphite, revealing a sequence of ordered and fluid phases consistent with experimental observations.

Contribution

It provides detailed simulation-based insights into the phase behavior of quantum hydrogen films on graphite, identifying specific transition points and structures without adjustable parameters.

Findings

Identification of a commensurate $\sqrt{3} imes \sqrt{3}$ solid phase near 1/3 coverage

Discovery of a uniaxially compressed incommensurate solid with domain-wall structures

Observation of phase transitions from solid to fluid states at specific temperatures and coverages

Abstract

We have used the Path Integral Monte Carlo method to simulate a monolayer of molecular hydrogen on graphite above 1/3 submonolayer coverage. We find that at low temperature and as the coverage increases the system undergoes a series of transformations starting from the $\sqrt{3} \times \sqrt{3}$ commensurate solid near 1/3 coverage. First, a phase is formed which is characterized by a uniaxially compressed incommensurate solid with additional mass density modulations along the same direction which can be viewed as an ordered domain-wall solid with a characteristic domain-wall distance which depends on the surface coverage. At low temperature and higher coverage there is a transition to an incommensurate solid which is rotated relative to the substrate commensurate lattice. As a function of temperature the domain-wall ordering first melts into a fluid of domain-walls and at higher temperature the solid melts into a uniform fluid. Regardless of the large amplitude of quantum fluctuations, these phase transitions are analogous to those in classical monolayer films. Our calculated values for the surface coverage and temperature where these transitions occur, the calculated structure factors and specific heat are in general agreement with the available experimental results with no adjustable parameteters.