By Eugenie Samuel Reich

Supersolids--bizarre quantum solids that flow effortlessly, as they have no friction--have come back into the limelight. The first claim to have made one, in 2004, was cast into doubt in June this year by experimental results suggesting that effects attributed to supersolidity might actually result from a different quantum phenomenon.

But backers of the supersolid interpretation are now poised to bounce back, with more definitive evidence of supersolid behaviour in a crystal of ultracold helium-4.

If its existence is confirmed, supersolidity will rank alongside superconductivity and superfluidity as a rare example of a quantum effect on a macroscopic scale.

The effect is similar to superfluidity, in which a fluid loses all viscosity, but in a solid material. "Supersolidity is a mysterious phenomenon, and it is controversial," says Sébastien Balibar, a physicist at the École Normale Supérieure in Paris. No comprehensive theory of the effect exists, and differing experimental results from several groups suggest that the final picture will not be simple.

Go with the flow

Eun-Seong Kim and Moses Chan at Pennsylvania State University in University Park claimed in 2004 to have created a supersolid. They based their assertion on an experiment in which they placed helium-4 in a torsional oscillator -- a vessel that continually rotates first one way and then the other. The oscillations increased in frequency when the temperature was reduced to less than two-tenths of a degree above absolute zero.

This was consistent with the idea suggested by Russian theoretical phyicists Alexander Andreev, now at the Kapitza Institute for Physical Problems in Moscow, and Ilya Lifshitz in 1969 that part of the helium-4 crystal might become supersolid at low temperatures. Unencumbered by friction, a supersolid would no longer be dragged around by such an oscillator, leaving the vessel freer to rotate. The free rotations would take less time, and so increase the frequency of oscillation.

Other groups have replicated Kim and Chan's observations but, in June, John Reppy, a physicist at Cornell University in Ithaca, New York, suggested that the effects might not indicate supersolidity. Reppy found that the changes in the frequency of oscillation occurred mostly at higher temperatures, when a supersolid would not be expected to form--because the quantum effect would be disrupted by the jiggling of the atoms--and that it increased when imperfections were introduced to the helium-4 crystal.

He suggested that movement of the imperfections within the crystal caused it to soften--an effect termed 'quantum plasticity' by John Beamish, a physicist at the University of Alberta in Edmonton, Canada--which could mimic the changes in the frequency of oscillation that had been ascribed to supersolidity.

The return of the theory

Now, in the latest volley in the debate, two groups have circulated unpublished results showing what they say are further quantum effects arising when torsional oscillators containing solid helium-4 are put into a rotating cryostat.

Masahiko Yagi, Minoru Kubota and their colleagues, physicists at the University of Tokyo, noticed that the oscillations subsided more quickly when the apparatus was rotated in the cryostat than when the cryostat was still.

Further results come from Kim, now at the Korea Advanced Institute of Science and Technology in Daejeon, South Korea, and his colleagues including Kimitoshi Kono from the Low Temperature Physics lab in Wako, part of Japan's network of research labs known as RIKEN. They have found that the observed increase in frequency of oscillations, ascribed to the formation of the supersolid state, was less marked when the cryostat was rotated than when it was still.

Both groups say that their findings could be caused by vortices in the supersolid helium-4, formed as a result of the rotation. The vortices would effectively restore some of the inertia that is missing in the supersolid state, because they would rotate with the rest of the oscillator.

"It is almost impossible to understand unless this is a supersolid," says Beamish. Plasticity, for instance, wouldn't be expected to respond to overall rotation of the apparatus.

Away from the oscillators

Reppy is not convinced by the new results, saying that the formation of vortices may not be the only explanation for the change in frequency observed by Kim's group. He says that he would like to see data on the oscillations of the empty vessel before he could be sure that vibration isn't causing part of the signal.

Balibar says that there could be both a supersolidity and a quantum plasticity effect in solid helium-4, and that the latter predominates in Reppy's experiments--perhaps because his apparatus is less rigid than that of other groups.

There is a growing consensus in the community of helium-4 researchers that supersolidity is real, says Humphrey Maris, a researcher into low-temperature physics at Brown University in Providence, Rhode Island. He adds that the debate over results from torsional oscillators makes it a priority for researchers to study helium-4 using other set-ups.

Michael Ray and Robert Hallock, condensed-matter physicists at the University of Massachusetts, Amherst, have done just that. They report that at low temperatures, atoms of fluid helium-4 can flow through a chamber filled with solid helium-4. As torsional oscillator experiments produce evidence for the disappearance of friction, but not directly for flow, Chan says that until this experiment, there was no evidence that there could be flow in solid helium.

Chan says that he did not expect so many questions to remain about supersolidity nearly a decade after he and Kim first saw evidence for it. "There are some puzzles but it is coming together... finally," he says.