Properties of Vacancy Formation in He-4 hcp Crystals at Zero Temperature and Fixed Pressure

TL;DR

This study uses diffusion Monte Carlo to analyze vacancy formation in He-4 hcp crystals at zero temperature, revealing how vacancy properties vary with pressure and correlating with supersolid behavior.

Contribution

It provides the first detailed quantum Monte Carlo calculations of vacancy formation enthalpy and volume in He-4 crystals across a range of pressures.

Findings

Minimum formation energy at 57 bar (~10.5 K)

Maximum vacancy formation volume at 57 bar, matching unit cell volume

Correlation between vacancy properties and supersolid fraction peak

Abstract

Equation of state of He-4 hcp crystals with vacancies is determined at zero temperature using the diffusion Monte Carlo technique, an exact ground state zero-temperature method. This allows us to extract the formation enthalpy and isobaric formation energy of a single vacancy in otherwise perfect helium solid. Results were obtained for pressures up to 160 bar. The isobaric formation energy is found to reach a minimum near 57 bar where it is equal to $10.5\pm 1.2$ K. At the same pressure, the vacancy formation volume exhibits a maximum and reaches the volume of the unit cell. This pressure coincides with the pressure interval over which a peak in the supersolid fraction of He-4 was observed in a recent experiment.