In the 15 January 2004 issue of the journal Nature, two physicists from Penn State University will announce their discovery of a new phase of matter, a "supersolid" form of helium-4 with the extraordinary frictionless-flow properties of a superfluid. "We discovered that 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. "We apparently have observed, for the first time, a solid material with the characteristics of a superfluid."
"The possible discovery of a new phase of matter, a supersolid, is exciting and, if confirmed, would be a significant advance," comments John Beamish, professor of physics at the University of Alberta and the author of a review of Chan's discovery published in the "News and Views" section of Nature. "If the behavior is confirmed, there are enough questions to be answered about the nature and properties of supersolid helium to keep both experimentalists and theorists busy for a long time."
Chan and graduate student Eun-Seong Kim made this discovery by using an apparatus that allowed them to compress helium-4 gas into a sponge-like glass disk with miniature atomic-scale pores while cooling it to almost absolute zero (-459.67 degrees Fahrenheit). The porous glass was inside a leak-tight capsule, and the helium gas became a solid when the pressure inside the capsule reached 40 times the normal atmospheric pressure. Chan and Kim continued to increase the pressure to 62 atmospheres. They also rotated the experimental capsule back and forth, monitoring the capsule's rate of oscillation while cooling it to the lowest temperature.
"Something very unusual occurred when the temperature dropped to one-tenth of a degree above absolute zero," Chan says. "The oscillation rate suddenly became slightly more rapid, as if some of the helium had disappeared." However, Chan and Kim were able to confirm that the helium atoms had not leaked out of the experimental capsule because its rate of oscillation returned to normal after they warmed the capsule above one-tenth of a degree above absolute zero. So they concluded that the solid helium-4 probably had acquired the properties of a superfluid when the conditions were more extreme.
Chan offers an analogy for understanding the results of this experiment. "Imagine that you are pushing a child on a swing. The friction of the child's weight couples him to the seat of the swing and he swings back to you at a certain rate, which is determined partly by the combined weight of the child and the seat. But if the child suddenly becomes able to hover above the swing instead of being directly connected to it, the overall weight of the swing's seat would become lighter and it would fly back to you at a faster rate," he explains. The researchers conclude that what happened inside their experimental capsule is that the tightly packed helium-4 particles became so slippery that they were no longer coupled to the walls of the glass sponge's micropores; in other words, it became a supersolid.
Beamish notes that, although superfluids are rare, they "play a fundamental role in fields as diverse as statistical mechanics and fluid dynamics and they provide a valuable testbed for applications ranging from turbulence to cosmology."
Chan says one way to think about the phenomenon of superfluidity is to imagine that each particle of helium-4 is a person standing on an overcrowded subway train at rush hour. "The door opens and some of the people want to move out, but they are packed so tightly together that there is a lot of friction between them. Under normal conditions, the people who want to stay on the train will be dragged out along with those who are pushing to get out the door. But if the packed subway riders somehow became infinitely slippery, they would flow like a superfluid--each moving person gliding with ease around those who were standing still," he explains. In other words, superfluids flow with no friction at all.
To understand how a supersolid could exist, you have to imagine the realm of quantum mechanics, the modern 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 like electrons and atoms with an odd mass number, like helium-3. Bosons include atoms with an even mass number, like 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 supercooled 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 explains. "In a supersolid of helium-4, its identical helium-4 atoms are flowing around without any friction, rapidly changing places--but because all its particles are in the identical quantum state, it remains a solid even though its component particles are continually flowing."
Chan and Kim tested their conclusion by performing the experiment again, but this time with the fermion helium-3, which theoretically is incapable of forming a supersolid. In this experiment, they found that there was no change in the oscillation period, even when the helium-3 was cooled to just 0.02 degrees above absolute zero--in stark contrast to the results with helium-4. "This control experiment with helium-3 gives more weight to our conclusion that the helium-4 in our experiment appears to have become a supersolid," Chan says.
If Chan's experiment is replicated, it would confirm that all three states of matter can enter into the "super" state, known as a Bose-Einstein condensation, in which all the particles have condensed into the same quantum-mechanical state. The existence of superfluid and "supervapor" had previously been proven, but theorists had continued to debate about whether a supersolid was even possible. "One of the most intriguing predictions of the theory of quantum mechanics is the possibility of superfluid behavior in a solid-phase material, and now we may have observed this behavior for the first time," Chan says.
Chan says his lab is interested in learning more about the thermodynamic, acoustic, and other properties of supersolid helium-4.
This research was supported by the Condensed Matter Physics Program of the National Science Foundation.
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