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Accueil du site > Equipes de recherche > Atomes dans monocristaux > recherche > Vers une transition supersolide dans l’hélium ?
Vers une transition supersolide dans l’hélium ?
Crystalline order and superfluidity
Andreev and Lifshitz suggested in 1969 that a quantum solid may well have its nodes not all occupied by atoms, and that the resulting vacancies could form a quantum gaz confined within the crystal. This gaz might in some circumstances undergo a Bose-Einstein Condensation, and confer to the crystal the ability to flow whithout viscosity, a property dubbed as "supersolidity".
Vacancies in solid helium
Vacancy energy in solid helium are deduced either from the dependance of the mobility of impurities with temperature, or from X-ray determinations of the lattice parameter of the crystal. Although measurement results are somewhat scattered, they all indicate a large dependence in the molar volume of the crystal when it is compressed above the solidification pressure.
- Vacancy energy vs molar volume in solid helium. Energy seems to vanish for a molar volume of 22 cm3 [B.A.Fraass et al. PRB 96, 105302 (1989)]
Minimum energy at equilibrium is of the order of 8K. This explains their vanishing density at low temperature. However, one might ask if this energy would eventually vanish were the molar volume further augmented past its equilibrium value up to, say, V
=22 cm
.
The elastic constants of solid helium have been measured as well at various molar volumes. A naive extrapolation seem to indicate that there should be no crystal instability (that is vanishing elastic constant) before one reaches a molar volume at least on the order of 23 cm. Hence, the helium crystal is expected still to be metastable at the interesting molar volume V.
- Compliances of hcp solid heluim vs molar volume. There seems to be a range of molar volume above equilibrium for which the solid should remain stable
Less naive extrapolation of existing EOS even suggests that its stability limit (vanishing rigidity) is reached at much higher molar volume.
We want to investigate the vacancy energy in stretched solid helium using two complementary techniques :
- using focused sound waves to put the solid locally at a pressure below the equilibrium melting pressure. This technique has been developed for liquids at LPS by the group of Sébastien Balibar with which we have a collaboration.
- using optical spectroscopy to measure vacancy energy. We hope that rare-earth atoms will provide us such a tool.
