At temperatures close to absolute zero, something amazing happens to helium-4; it collapses into a superfluid, a frictionless liquid that can flow forever. In the 15 January issue of Nature, a team reports something even stranger: solid helium-4 that flows like a superfluid. If the observations are correct, physicists have identified a new, "supersolid" state of matter.
To make a superfluid, individual atoms (or other particles, in theory) must be so cold that they all drop into the lowest possible energy state. The atoms don't whiz around vibrating and jostling the way warmer particles do, so there's no friction between them. Superfluidity has been observed in both liquids and gases, but researchers have tried to make superfluid solid helium for more than 20 years without success.
Moses Chan and Eun-Seong Kim of Pennsylvania State University, University Park, suspected that the barrier to superfluidity lay in the perfection of the lattice-like arrangement helium atoms assume in solid form. Chan and Kim reasoned that holes in this structure might help the helium flow, by allowing the atoms to change places with the holes as they move through the lattice. To put this idea to the test, they used pressures of about 60 atmospheres to force helium-4 to solidify within the wormlike tunnels of a porous glass. Due to the tininess of the tunnels and the mismatch between the molecular structures of the glass and the helium-4, the helium solidified within many tiny holes.
The researchers then spun the glass disk containing the solid helium, measuring the disk's inertia as they lowered the temperature. Below 175 millikelvins, its inertia suddenly dropped. This would make sense if the helium had become superfluid and was flowing frictionlessly without dragging on the disk as it turned. Another possibility was that some of the helium-4 leaked out, but when the temperature rose above 175 mK, the inertia returned to its original level, ruling out a helium leak.
It's a "sensational" result, says Bill Halperin of Northwestern University in Evanston, Illinois. "The new supersolid state has been a subject of intense interest and speculation for many years."