Bose-Einstein Condensation (BEC) was first achieved in the liquid phase in helium a century and, for gas phase atoms, a decade ago. The question arises if there could be a third BEC in a solid, crystalline, state, as predicted by Lifshitz: the “Supersolid”.
We are working on a possible pathway towards such a new state of matter: a quantum plasma for which the electron de Broglie wavelength becomes larger than the mean distance between particles, the Wigner-Seitz radius. For the electrons in an ultra-cold ion-electron plasma the electronic degeneracy condition is fulfilled for a temperature below 0.1K and a density above 1015 cm3. We produce such an ultra-cold Rydberg plasma by laser threshold ionization of NO molecules in the high-density expansion region of a supersonic jet close to the nozzle. This plasma has an extremely long lifetime of milliseconds and it shows the compressibility of a “sponge like” ultra-soft solid. An explanation of our experimental measurements is the formation of a degenerate electron fluid, which should lead to the formation of a lattice of the cations, an Abrikosov/Wigner ion crystal. Such a crystal would probably be far from perfect and exhibit a large number of defects. These crystal vacancies could then form a Bose-Einstein condensate producing a “Supersolid”.