Investigating metastable hcp solid helium below its melting pressure

TL;DR

This study explores creating metastable hcp solid helium below its melting pressure using focused sound pulses, revealing the potential to induce negative pressure states and analyzing wave behavior in anisotropic crystals.

Contribution

First experimental attempt to produce metastable hcp solid helium below melting pressure using focused sound pulses and optical interferometry.

Findings

Negative pressure swings up to 0.9 bar achieved

Wave splitting due to crystal anisotropy observed

Focusing amplification remains significant

Abstract

We report a first attempt to produce metastable hcp solid helium below its melting pressure. A focused sound pulse is emitted along the c-axis of a mono-domain hcp helium-4 crystal starting from a static pressure just above the melting pressure. The sound pulse is made as simple as possible with one negative and one positive swing only. Density at focus is monitored by an optical interferometric method. Performed numerical simulations show that the crystal anisotropy splits the focused wave into two separate pulses, corresponding to a longitudinal wave along the c-axis and a radial one perpendicular to it. The amplification factor due to focusing remains nevertheless important. Negative pressure swings up to 0.9 bar have been produced, crossing the static melting pressure limit. Improvements in the detection method and in the focusing amplification are proposed.