The Limit of Mechanical Stability in Quantum Crystals: A Diffusion Monte Carlo Study of Solid 4He

TL;DR

This study uses diffusion Monte Carlo methods to determine the mechanical stability limit of solid helium-4, revealing it occurs at a pressure significantly below the liquid phase spinodal, with unique sound velocity behaviors near this limit.

Contribution

First-principles calculation of the mechanical stability limit of solid helium-4, providing new insights into its elastic and sound velocity properties at negative pressures.

Findings

Mechanical stability limit at -33.82 bar

Stability limit below liquid phase spinodal at -9.6 bar

Non-power-law behavior of sound velocities near stability limit

Abstract

We present a first-principles study of the energy and elastic properties of solid helium at pressures below the range in which is energetically stable. We find that the limit of mechanical stability in hcp 4He is $P_{s}$ = -33.82 bar, which lies significantly below the spinodal pressure found in the liquid phase (i.e., -9.6 bar). Furthermore, we show that the pressure variation of the transverse and longitudinal sound velocities close to $P_{s}$ do not follow a power law of the form $\propto \left( P - P_{s} \right)^{\gamma}$, in contrast to what is observed on the fluid.