A Likely New Supersolid Phase of Matter

Article excerpt

In a recent issue of Science Express, two physicists from Penn State University announced new experimental evidence for the existence of a new phase of matter, a "supersolid" form of helium-4 with the extraordinary frictionless-flow properties of a superfluid.

"Solid helium-4 appears to behave like a superfluid when it is so cold that the laws of quantum mechanics govern its behavior," says Moses H.W. Chan, Evan Pugh Professor of Physics at Penn State.

"One of the most intriguing predictions of the theory of quantum mechanics is the possibility of superfluid behavior in a solid, particularly solid helium-4, and we have strong experimental evidence for this behavior," Chan says.

Chan, and his former student and current postdoctoral associate Eunseong Kim, first announced in the January 15, 2004, issue of the journal Nature their observation of the superfluid-like behavior of solid helium-4, which they had confined in a porous glass with pore diameters of several nanometers.

In their current experiment, they observed the same superfluid-like behavior in samples of bulk solid helium without any confining matrix.

"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," Chan explains.

Nobel laureate Anthony Leggett, who comments on Chan's discovery in the "Perspectives" section of the journal Science, illustrates the concept of a supersolid by saying, "Imagine you take a small solid body--say a coin--set it on the axis of an old-fashioned gramophone turntable, and set the latter into slow rotation. Then the coin will rotate with the turntable--won't it? Not if it is made of solid 4-He [helium-4] ..."

Such a failure to rotate is characteristic of a superfluid and is known as nonclassical rotational inertia, or NCRI.

"Leggett says of Chan's latest research, "... the most plausible interpretation, and the one drawn by the authors, is that NCRI is indeed occurring . . ."

As in their earlier experiment, Kim and Chan used a laboratory device called a torsional oscillator, which is like an amusement-park ride for experimental samples that rapidly rotates back and forth, to study the rotational property of solid helium. The helium is contained inside a ring-shaped, or "annular," channel located inside the sample cell. The researchers introduce helium gas into the open annular channel under high pressure via a thin capillary tube.

Solid helium forms in the channel when the cell is cooled below -270 degrees Celsius, or 3 degrees above absolute zero, under a pressure that exceeds 26 times the normal atmospheric pressure. Kim and Chan then rotated the sample cell back and forth while cooling it to the lowest temperature.

"Something very unusual occurred when the temperature dropped below one- quarter of a degree above absolute zero," Chan says. "The oscillation rate suddenly became slightly more rapid, as if some of the helium has disappeared or simply was not participating in the torsional motion."

Kim and Chan found it easy to confirm that the helium had not disappeared--they just warmed the experimental cell and found the oscillation returned to the same slower rate. "The sensible interpretation of the result is that some of the helium does not participate in the oscillation," Chan explains. …