Observation of metastable hcp solid helium.
Résumé
We have produced and observed metastable solid helium-4 below its melting pressure near $1.1$ 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. Minimum density achieved at focus corresponds to a pressure about 2~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.
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