Observation of metastable hcp solid helium

TL;DR

This study demonstrates the creation and observation of metastable solid helium-4 below its melting pressure using focused pressure waves, revealing earlier-than-expected instabilities that open new research avenues.

Contribution

It introduces a method to produce and analyze metastable solid helium-4 with optical density mapping, revealing unexpected stability limits.

Findings

Metastable solid helium-4 achieved below melting pressure.

Instability occurs earlier than predicted spinodal limit.

Optical interferometry used for precise density mapping.

Abstract

We have produced and observed metastable solid helium-4 below its melting pressure between 1.1 K and 1.4 K. This is achieved by an intense pressure wave carefully focused inside a crystal of known orientation. An accurate density map of the focal zone is provided by an optical interferometric technique. Depending on the sample, minimum density achieved at focus corresponds to pressures between 2 and 4 bar below the static melting pressure. Beyond, the crystal undergoes an unexpected instability much earlier than the predicted spinodal limit. This opens a novel opportunity to study this quantum crystal in an expanded metastable state and its stability limits.