Where did the supersolidity go?

Helium is a rich source of exotic phenomena. Below 2.2 K, helium-4 exhibits superfluidity, a manifestation of Bose–Einstein condensation. And ⁴He doesn’t solidify at any temperature, except under very high pressure. Lattice vacancies in solid ⁴He are thought to remain mobile down to absolute zero. Therefore, theorists have long suggested that quantum condensation of those vacancies might create a ‘supersolid’ phase in which atoms flow without hindrance or dissipation. The first apparent sighting of supersolidity came in 2004. Moses Chan’s group at the Pennsylvania State University reported an abrupt drop in the period of a torsion oscillator (shown in the photo) when its frozen ⁴He sample was cooled below 0.2 K. The drop was attributed to a decrease of the sample’s moment of inertia as about 1% of it transitions into a supersolid and thus decouples from the oscillation. Numerous later experiments by groups worldwide found similar results. But following the 2007 discovery of an abrupt rise in the elastic stiffness of bulk solid ⁴He just at the presumed transition temperature, the supersolid interpretation has been in doubt. However, because Chan’s 2004 experiment was meant to confine the solid ⁴He within the nanopores of a porous glass, the result should have been immune to the stiffening of bulk solid ⁴He. But that design might have allowed inadvertent thin films of solid ⁴He outside the glass to stiffen the oscillator. So now the group has redone the old experiment with a new sample-holder design that eliminates the possibility of such films. And this time they find no evidence of a transition to supersolidity. (D. Kim, M. Chan, Phys. Rev. Lett., in press, available at http://arxiv.org/abs/1207.7050 .)—Bertram Schwarzschild