Science News - September 11, 2010

at temperatures where supersolidity could be happening — 80 millikelvins. “You could interpret our experiments as that we are seeing flow associated with what might be expected for supersolid behavior,” Hallock says.

Still, he adds, “I’m much more cautious than that.” In part that’s because of what happens when the researchers lower the temperature below 80 millikelvins: The rate of flow drops dramatically, the oppo site of what might be naïvely expected if supersolidity were occurring. And the flow occurs at much higher temperatures as well, up to 600 millikelvins. “Some thing is necessary to explain the ability of our experiment to pass atoms through the cell,” says Hallock. “Is it a supersolid? Could be. Is it for certain? I don’t know.” Some theorists think that Hallock’s work is the closest to demonstrating true supersolidity. Anatoly Kuklov, a theo retical physicist at the City University of New York’s College of Staten Island, says the experiment shows that super solidity exists, but probably not the kind originally envisioned in 1969. Instead of a simple shifting around of vacancies within a crystal, he says, supersolidity could occur because of atoms moving along a different kind of crystal defect, called a dislocation. Such defects, known for decades, could offer a new explana tion for supersolidity’s bizarre behavior.

To picture a dislocation, imagine a book in which one page has been ripped in half horizontally. Look at the closed book from the bottom, and all pages might appear to be intact. But look at it from the top, and one page will appear to be missing. The line of that missing page is analogous to a crystal dislocation.

Numerical simulations of a helium crystal with no such dislocations showed no signs of supersolidity, Kuklov says. And experimental results from differ ent laboratories vary widely depending on how the crystals are prepared — again suggesting that the purity of the crystals could be important. “So the effect is most likely produced,” Kuklov says, “by a net work of defects which can conduct flow,” in which atoms move along the defects and allow the solid to flow like a fluid.

Source froM left: a.r. barron and c. SMith/ coNNexioNs 2010; ndt education reSource center

The role of defects Many researchers think that supersolid behavior may arise because of the motion of atomic defects through a solid’s crystal lattice. in one common type of defect, called a vacancy, an atom is simply missing in the lattice (left). in another, called a dislocation, an entire atomic row is truncated, causing the rest of the structure to warp accordingly.

To test this idea, experiments are underway to see what happens to a single perfect crystal of helium, free of defects, in a torsional oscillator. Chan and Sébastien Balibar, of the École Normale Supérieure in Paris, have built a contrap tion where the oscillator’s sides are made of transparent sapphire, providing a win dow to see what’s going on inside. (Other torsional oscillators use metallic contain ers that obscure the view within.) Inside the researchers put the purest crystal they can make. As it oscillates, new dis locations pop into existence.

At a supersolidity workshop in Paris in late July, Balibar and Chan reported seeing single dislocations move “like vio lin strings” through the solid helium at rates of up to several meters per second. Such speed is impossible with everyday materials and could occur only if quan tum phenomena like supersolidity were in play, Balibar says.

In a paper to appear in Physical Review Letters, Balibar and colleagues also report that these vibrating dislo cations can cause ultrapure crystals of solid helium to soften. Oddly, super solidity appears only when impurities in the crystal prevent the dislocations from moving.

Additional new evidence supporting
supersolidity was reported at the Paris
meeting by Kim—now at the South
Korean university KAIST — and his col
league Kimitoshi Kono of the RIKEN
research institute in Tokyo. They took a
torsional oscillator and, as it swung back
and forth, also rotated it around its axis
like a spinning Earth. “For almost any
classical metallurgical explanation, one
that doesn’t involve superfluidity, it’s
hard to imagine how rotating it would
make any difference,” says Beamish. “It’s
a fundamental property of superfluids
that rotation makes a huge difference.”
And that’s exactly what Kim reported at
the conference — that mass was flowing
through itself not only from the back and
forth of the oscillator but also because of
the added rotation.