Physicists at Penn State University, University Park, Pa., announced experimental evidence for the existence of a new phase of matter, a “supersolid” form of helium-4 with the frictionless-flow properties of a superfluid. The term “superfluid” was coined in 1938 by the Soviet physicist Pyotr Kapitsa. His experiments were based on helium-2, and showed an apparent enormous rise in heat conductivity, rapid flow through capillaries or over the rim of the containment vessel as a thin film, as well as other unusual properties.
Moses H.W. Chan, professor of physics, and his associate, Eunseong Kim, made an initial announcement earlier this year based on experiments where solid helium-4 was confined in porous glass having pore diameters of several nanometers. In the current experiment, they observed the same superfluid-like behavior in samples of bulk solid helium without any confining matrix. “Solid helium-4 appears to behave like a superfluid when it is so cold that the laws of quantum mechanics govern its behavior,” Chan says. “Our current experiments with bulk solid helium indicate that the superfluid-like behavior we observed is an intrinsic property of the solid — not the result of confinement in any particular porous medium and not a consequence of the large surface area that accompanies a porous host.”
When liquid helium is cooled below 2.176K, it exhibits the frictionless-flow properties of a superfluid and can flow through atomic-size pores. “When the temperature dropped below one-quarter of a degree above absolute zero, the oscillation rate suddenly became slightly more rapid, as if some of the helium had disappeared or simply was not participating in the torsional motion,” Chan says. Kim and Chan simply warmed the experimental cell to find that the oscillation returned to the same slower rate, indicating the helium was still there. “The sensible interpretation of the result is that some of the helium does not participate in the oscillation,” Chan explains. “In other words, solid helium does not behave as an ordinary solid, but exhibits nonclassical, or reduced, rotational inertia in the supersolid phase.”
The researchers conclude that what happened inside their experimental sample cell is that a small fraction — roughly 1.5% — of the helium atoms enter into a state of zero friction and that this fraction is no longer coupled to the back-and-forth motion of the sample cell or to the rest of the solid. “This 1.5% is the supersolid fraction, and its behavior is identical to that found for liquid helium entering the superfluid phase, except that in liquid helium the superfluid fraction is 100% at absolute zero,” Chan says. Kim and Chan found supersolid behavior in 17 different samples of solid helium at pressures ranging from 26 atmospheres up to 66 atmospheres.
Although there is no practical known use for superfluid properties, the results of the experiments force physicists to do some thinking. “We used to think that a solid could not flow, but now we have discovered that when you cool solid helium to a sufficiently low temperature it can not only flow, but it actually flows without friction,” Chan says. “The implication of our research is that we now have to rethink what we mean by a solid.”