A liquid ground state for 2D helium-3?

Because helium-3 is so light, and because as a fermion it can't crowd into low energy levels, it has an unusually large ground-state energy. When that energy is partitioned among two dimensions instead of three, it's thought to be large enough to overcome induced-dipole attractions and prevent condensation into a two-dimensional liquid, even at absolute zero. That's why new results from a University of Tokyo group led by Hiroshi Fukuyama are so puzzling: In experiments with ultracold ³He adsorbed on graphite—a setup that should closely approximate a genuine 2D quantum system—the researchers found that under certain conditions, atoms appeared to collect into liquid puddles of fixed, low density, as depicted in the illustration. Previous experiments with ³He atoms adsorbed on thin films of superfluid ⁴He yielded similar hints of puddle formation, but the Tokyo study is the first to confirm that the condensed phase isn't an artifact of 3D or hydrodynamic effects. If puddles of 2D liquid ³He are indeed forming, they would be the lowest-density liquid ever found in nature, with a mean interatomic distance of more than 1 nm, or nearly 30 atomic radii. (D. Sato et al., Phys. Rev. Lett. 109, 235306, 2012.)—Ashley G. Smart