Observation of metastable hcp solid helium (Vol. 42, No. 6)

Crystalline solids can be brought into metastable state with respect to fusion only if surface melting is avoided. Overheated metals have indeed been observed by embedding small samples in carefully chosen matrices. Because of its constant melting pressure at low temperatures, hcp solid helium offers a unique possibility to achieve a metastable solid via pressure variations. Intense positive and negative pressure swings far from any interface can be achieved using focused sound waves. In hcp solid helium, the sound velocity is anisotropic and a dedicated non-spherical sound emitter has to be used. The wave amplitude is small enough not to melt the crystal at its interface with the emitter. As it propagates, its amplitude increases and pressures below the static melting line are obtained in the solid bulk. The pressure is measured via the refraction index changes of the medium using an interferometric imaging technique. The main result of this work is shown on the figure: hcp solid helium between -2 and -4 bars below the melting line has been produced and observed. A side result is that the crystal seems to become unstable beyond this value. We feel that the stretched quantum solid is an interesting new system to be understood in details.