A Solid Like No Other: Frigid, solid helium streams like a liquid

Captured within the cavities of a porous glass disk, frozen helium has coalesced into a long-awaited, but never-before-observed, quantum phase of matter, a team of physicists claims. In that extraordinary state, known as a superfluid solid or supersolid, the material is expected to flow like a liquid yet maintain its solid crystal structure, says team leader Moses H.W. Chan of Pennsylvania State University in State College.

COLD CAPSULE. At ultracold temperatures, gyrations of this aluminum bob (arrow) indicate that the frozen helium inside it flows without friction. Penn State

Frictionless flow, also known as superfluidity, has previously been observed only in liquids and gases (SN: 10/25/03, p. 262: Available to subscribers at Super Spinner: Seven-atom speck acts like superfluid). “Now, we’re saying that even in a solid we can see it,” Chan says. In the Jan. 15 Nature, he and his Penn State colleague Eun-Seong Kim present evidence for what they suspect is the world’s first supersolid.

“If this discovery of a supersolid is confirmed, it is a major advance,” says John R. Beamish of the University of Alberta in Edmonton in a commentary accompanying the report.

William P. Halperin of Northwestern University in Evanston, Ill., calls the new evidence for supersolidity a “sensational result.”

For more than 30 years, theorists have predicted supersolidity, but experimentalists had been unable to demonstrate it. Building on a previous approach, Chan and Kim entered the fray with a device called a torsional oscillator. Basically, it’s a squat, cylindrical bob suspended from a hollow copper tube that slowly gyrates back and forth. To explore the behavior of solid helium, the researchers placed inside the oscillator’s bob a porous glass disk the diameter of a dime.

Then they infiltrated the disk’s pores with liquid helium and froze the helium under pressure at temperatures near absolute zero.

As the scientists continued to lower the temperature, they detected signs that at about 175 millikelvins the solid version of the isotope helium-4 stopped being dragged around because of friction with the disk. “Ordinarily, you would not imagine that this solid could become unstuck . . . but all the data indicate that, indeed, this helium did become unstuck,” Chan says. For the solid to act this way, atoms and vacancies within its crystal lattice may be continuously exchanging places.

Solid helium’s superfluidity indicates that a so-called Bose-Einstein condensate forms within the material. In such a condensate, atomic or subatomic particles share the same quantum state, amassing into what is, in essence, a single superparticle.

Since 1995, physicists have made numerous gaseous Bose-Einstein condensates from atoms of certain metals and from some simple molecules (SN: 11/22/03, p. 324: Quantum Pileup: Ultracold molecules meld into oneness). Decades earlier, isotopes of helium were discovered to form liquid Bose-Einstein condensates. Now, helium-4 has become the first solid Bose-Einstein condensate, according to the new experiment.

Vacancies in a crystal can behave like particles, Halperin explains. In supersolid helium, the particles forming the Bose-Einstein condensate are probably not the helium atoms but the lattice vacancies, he says.

Anthony J. Leggett of the University of Illinois at Urbana-Champaign (SN: 10/11/03, p. 229: Available to subscribers at Nobel prizes go to scientists harnessing odd phenomena), one of the first theorists to propose supersolidity, doubts that Chan and Kim have uncovered the supersolidity that he and other theorists have envisioned. After all, he notes, helium frozen within glass pores differs markedly from the bulk crystals to which the theories apply. Instead, he speculates that the scientists have detected some other type of superfluid behavior.

Chan says that his group and others are preparing experiments to learn whether frozen helium has other expected supersolid properties, such as a spike in its capacity to absorb heat and long-lasting movement unfettered by friction once the solid is set in motion.


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