"Ghostly, flowing supersolid? No, it's quantum plastic." Or "puny flaps."

This "solid that flows, ghostlike, through itself" relates beautifully to Samuel Taylor Coleridge's "fluttering stranger" from Frost at Midnight, I must say.

New Scientist
30 June 2010 by Eugenie Samuel Reich
Magazine issue 2767

Ghostly, flowing supersolid? No, it's quantum plastic.

IT'S one of the weirdest things predicted by quantum mechanics: a solid that flows, ghostlike, through itself. As if that's not enough to get your head round, experiments that claim to have made this "supersolid" may in fact have resulted in something completely different.

"We still do not understand the phenomenon. It's something new," says John Reppy, of Cornell University in Ithaca, New York, who claims to have glimpsed the new effect, dubbed quantum plasticity.

In a solid, atoms are bound together in a regular lattice, keeping their structure rigid under normal circumstances. But at certain temperatures supersolidity is thought to kick in. Cool some solids close to absolute zero, and they should become frictionless and flow like a liquid, while retaining their lattice structure.

In 2004, Moses Chan and Eun-Seong Kim, then at Pennsylvania State University in University Park, claimed to have produced a supersolid by cooling a cylinder of helium-4 to within a whisker of absolute zero.

They placed the cylinder so it oscillated around a central axis - rotating a short distance in one direction and then switching to the other. As they lowered the helium's temperature, Chan and Kim noticed that the cylinder was oscillating more slowly. They assumed this was due to a drop in the fraction of solid helium rotating along with the cylinder. As it is friction that causes the helium inside the cylinder to rotate when the cylinder itself rotates, the researchers attributed this drop to a decrease in the friction of the helium. They concluded that the supersolid effect had come into play at low temperatures (Science, DOI: 10.1126/science.1101501).

While Chan and Kim's results have been replicated, Reppy says their interpretation may be wrong. To probe supersolidity further, he added a flexible diaphragm to the top of the cylinder that allowed him to crush the helium, creating extra "defects" in its lattice. Previous experiments hinted these might enhance supersolidity, but Reppy found no evidence of this effect.

What's more, he found that as he raised the temperature above 200 millikelvin, the frequency of the oscillation decreased, though the transition to supersolidity isn't supposed to happen at these temperatures (Physical Review Letters, DOI: 10.1103/PhysRevLett.104.255301).

He concludes that the link between temperature and oscillation frequency is down to a totally new quantum effect, not supersolidity. This new effect, Reppy says, occurs due to the defects inherent in all solid helium-4, which change their behaviour at different temperatures.

Reppy reckons that as the temperature rises, the defects become more mobile, making helium's structure less rigid. This "wobbliness" slows down the oscillations. Because it is different to normal softness and is probably due to quantum effects, the phenomenon is called quantum plasticity. "It's different from normal plasticity," says Reppy.

We still do not understand the phenomenon of quantum plasticity, it's something new
He isn't ruling out the existence of supersolidity altogether but suggests that those who say they have seen it may in fact have been observing quantum plasticity.

Kim, now at KAIST in Daejeon, South Korea, disputes the notion that he and Chan misinterpreted their results but finds Reppy's findings intriguing.