Physicists find strong evidence of a new, supersolid phase of matter

University Park, Pa. -- In today's (Sept. 3) issue of Science Express , two physicists from Penn State announce 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," said 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, and his former student and current postdoctoral associate Eunseong Kim, first announced in the Jan. 15 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 explained.

Nobel Laureate Anthony Leggett, who comments on Chan's discovery in the Perspectives section of the journal Science, illustrated 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.

Regarding Chan's latest research, Leggett added "... 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 Celsius, or 3 degrees above absolute zero, under a pressure that exceeds 26 times the normal atmospheric pressure. Kim and Chan then rotate 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 sayid. "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 explained. "In other words, solid helium does not behave as an ordinary solid, but exhibits nonclassical, or reduced, rotational inertia in the supersolid phase, as described by Tony Leggett."

The researchers conclude that what happened inside the annular channel in their experimental sample cell is that a small fraction -- roughly 1.5 percent -- 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 percent 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 percent at absolute zero," Chan explained.

Kim and Chan found supersolid behavior in 17 different samples of solid helium at pressures ranging from 26 atmospheres up to 66 atmospheres.

"What seems certain is that if the interpretation Kim and Chan give of their raw data is correct (and quite probably even if it is not!), their experiment will force theorists to revise dramatically the generally accepted picture of crystalline solid 4-He," Leggett said.

To understand how a supersolid could exist, one has to imagine the realm of quantum mechanics, the theory that explains many of the properties of matter. In this realm there are different rules for the two categories of particles: fermions and bosons. Fermions include particles such as electrons and atoms with an odd mass number, such as helium-3. Bosons include atoms with an even mass number, such as helium-4. The quantum-mechanical rule for fermions is that they cannot share a quantum state with other particles of their kind, but for bosons there is no limit to the number that can be in the identical quantum state. This talent that bosons have for Rockettes-style coordination leads to the remarkable properties that Chan and Kim discovered in solid helium-4.

"When we go to a low-enough temperature, thermal energy is no longer important and this quantum-mechanical effect becomes very apparent," Chan explained. "In the supersolid phase, the supersolid fraction of the particles are executing Rockettes-style coherent superflow around the annular channel, as viewed by the oscillating sample cell."

Kim and Chan tested their conclusion by performing the experiment again, but this time they built a new sample cell with a barrier in the annular channel, blocking its continuous "racetrack" geometry so that superflow could not take place. "In this experiment, we observed that the decoupling rate, as measured by the change in the oscillation rate, decreased by a factor of 60," Chan reported. "The small residual effect is due to the special property of a superfluid and supersolid known as the irrotational flow effect. What is clear is that superflow is indeed interrupted by the barrier in the annular channel," Chan said.

In addition to Chan's group, a number of other labs and theoretical groups are gearing up to learn more about the thermodynamic, hydrodynamic, and other properties of supersolid helium-4.

"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 said. "The implication of our research is that we now have to rethink what we mean by a solid."

Chan's research is supported by the Condensed Matter Physics Program of the National Science Foundation.