Phase behaviour of the quantum Lennard-Jones solid

TL;DR

This study investigates the phase stability of quantum Lennard-Jones solids using advanced simulation techniques, revealing quantum effects favor hcp structures and suggesting potential quantum melting in noble gas solids like helium.

Contribution

It introduces a combined approach of thermodynamic integration, path integral molecular dynamics, and lattice dynamics to analyze quantum phase behavior of Lennard-Jones solids, a topic previously unexplored.

Findings

Quantum effects stabilize hcp phase in Lennard-Jones solids.

fcc phase is favored by lattice dynamics.

Quantum melting occurs for helium-like parameters at zero pressure.

Abstract

The Lennard-Jones potential is perhaps one of the most widely-used models for the interaction of uncharged particles, such as noble gas solids. The phase diagram of the classical LJ solid is known to exhibit transitions between hcp and fcc phases. However, the phase behaviour of the quantum Lennard-Jones solid remains unknown. Thermodynamic integration based on path integral molecular dynamics and lattice dynamics calculations are used to study the phase stability of the hcp and fcc Lennard-Jones solids. The hcp phase is shown to be stabilized by quantum effects in PIMD while fcc is shown to be favoured by lattice dynamics, which suggests a possible re-entrant low pressure hcp phase for highly quantum systems. Implications for the phase stability of noble gas solids are discussed. For parameters equating to Helium, the expansion due to zero-point vibrations is associated with quantum melting: neither crystal structure is stable at zero pressure.