Sommaire:

Condensed He-4 is a fascinating system because it exhibits quantum properties at the macroscopic scale. The most famous of these is probably the transition of the liquid into a so called superfluid state when cooled down below 2.17 K at vapor pressure. I will present and discuss experiments investigating the metastable phases of liquid and solid He-4. Achieving a metastable phase of (for instance) liquid 4 He consists in bringing a sample of liquid 4 He at a temperature and a pressure (or density) where the thermal equilibrium phase is not the liquid phase but rather the solid one or the gaseous one. We experimentally realize these states by focusing an acoustic wave (pressure/density wave) in the bulk of the liquid (solid). We measure the local density within the metastable state thanks to an optical time-resolved interferometric method. In both metastable liquid and solid He-4, we have observed instabilities at densities which are not compatible with theoretical predictions. These results are interesting because metastable liquid He-4 is a model system to study homogeneous cavitation and because solid metastable solid He-4 is a potential candidate for a hypothetical (not yet discovered) supersolid state of matter.

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