Atomic displacements in quantum crystals

TL;DR

This study uses Quantum Monte Carlo simulations to analyze atomic displacements in helium and parahydrogen solids at low temperatures, revealing quantum-mechanical behavior and thermal contributions near melting.

Contribution

It provides detailed quantum and thermal displacement analysis in helium and parahydrogen solids, with results aligning with experimental Lindemann ratios at melting.

Findings

Atomic displacements in helium are predominantly quantum-mechanical.

Lindemann ratio at melting agrees with experimental data.

Thermal contributions significantly affect molecular vibrations in parahydrogen.

Abstract

Displacements of atoms and molecules away from lattice sites in helium and parahydrogen solids at low temperature have been studied by means of Quantum Monte Carlo simulations. In the bcc phases of He-3 and He-4, atomic displacements are largely quantum-mechanical in character, even at melting. The computed Lindemann ratio at melting is found to be in good agreement with experimental results for He-4. Unlike the case of helium, in solid parahydrogen there exists near melting a significant thermal contribution to molecular vibrations, accounting for roughly half of the total effect. Although the Lindemann ratio at melting is in quantitative agreement with experiment, computed molecular mean square fluctuations feature a clear temperature dependence, in disagreement with recent experimental observations.