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

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.

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• Temperature Dependence of the Vacancy Formation Energy in Solid ${}^4$He
• A coordinated wavefunction for the ground state of liquid helium-4
• Quantum Phase Transition with a Simple Variational Ansatz