What state is a supersolid of Helium?

MAY 07, 2009

Physics Today

Science : Superfluid helium is best known for its ability to flow without resistance. Superfluids also differ from ordinary fluids in that they fail to respond to a slow steady rotation says John Saunders in Science magazine . The atoms in a superfluid are in the same quantum state, so they move coherently and cannot gradually “spin up,” as does water in a rotated container. An intriguing question is whether a supersolidâmdash;formed by applying pressure to a superfluidâmdash;could combine these remarkable properties, quantum coherence and dissipationless mass flow of atoms, in a solid that still has structural order and rigidity.

In an ordinary classical crystal, all atomic motion is frozen out at absolute zero, but solid helium-4 is a quantum solid; each atom is highly delocalized in a quantum probability cloud around its equilibrium position, and as a result, atoms on neighboring sites can exchange positions and move through the solid. In 2004, Kim and Chan (5) claimed to have observed supersolidity in solid helium-4. This discovery was followed by experimental and theoretical studies suggesting that disordered glassy solids play a key role in creating the putative super-solid state.

Two reports in last week’s Science journal address the origin and effects of this disorder. Hunt et al. report their observation of the onset of remarkable ultraslow dynamics on cooling samples of solid helium, which constitutes new evidence for glass-like behavior. They reveal a subtle interplay between this glassiness and the observed supersolid-like mechanical responses. Philip Anderson argues that the supersolid will still occur in a pristine crystal, but coupling to disordered regions near dislocations enhances the supersolid response. His bold hypothesis is that every solid composed of bosons will have a supersolid ground state.