Supersolidity loses its luster

One of the most exciting physics discoveries in recent years may not be a discovery after all. Reports of “supersolidity,” in which solid helium flows through itself without friction, may turn out be something far more ordinary: the everyday stiffening of a material.

This new conclusion comes from the same scientist who in 2004 first reported evidence for supersolidity. Now, in a paper published October 8 in Physical Review Letters, Moses Chan of Penn State says he has repeated that original experiment, eliminating more possible sources of experimental error and saw no hints of supersolidity.

“It would have been neat if the phenomenon holds up,” Chan says. But instead, he says, he feels “a sense of disappointment.”

A discovery of supersolidity would be the stuff Nobel Prizes are made of. Superfluids are quantum liquids that flow according to unusual rules, such as up and over the sides of a container; supersolids would be the solid equivalent, in which atoms somehow leave their crystalline lattice and flow effortlessly within it.

Chan’s original experiment, done with Eunseong Kim, looked for traces of how this supersolid flow might affect the twisting movement of a cylinder. They filled a glass cylinder with solid helium and set it twisting back and forth, like a merry-go-round going first one way and then the other. Chan and Kim chilled the apparatus down and saw the cylinder begin to twist a whole lot faster. They concluded that some of the helium had begun to flow as a supersolid, essentially releasing its hold on the material around it and causing the inertia — the resistance to change in motion — to drop.

Later, other similar experiments seemed to confirm this finding (SN: 9/11/10, p. 22). But big questions remained, such as why some lab groups saw a large effect and others a small one. “It was continually surprising to those of us working in the field just how hard it was to confirm or disprove the existence of supersolidity,” says John Beamish, a physicist at the University of Alberta in Canada. “Every year we expected everything to be clear very soon.”

Work by Beamish and others suggested that helium’s odd behavior happened not when the helium crystals were perfect, but rather when they were marred by structural imperfections that moved around within the material. It now looks as if that sliding causes the material to stiffen in ways that mimic (but are not) supersolidity.

In his original experiment, Chan did as much as he could to reduce such unwanted effects, but in the end he decided to redesign the cylinder from scratch to be sure. The problem turned out to be a small gap between the glass cylinder and a metal plate at the bottom of the container, where solid helium puddled and threw off the measurements. Scientists had thought Chan’s gap was too small to be much of a problem, but the effect is actually magnified in a small space, says Beamish.

The moving imperfections turn out to be a new quantum phenomenon of their own, says Sébastien Balibar, a physicist at the École Normal Supérieure in Paris. “Not only do we understand it now as a spectacular manifestation of a fundamental phenomenon in material science, it is also the most likely interpretation of the so-called ‘supersolidity,’ ” he says.

Several other recent studies had also suggested that stiffening, not supersolidity, might be at work. But a few experiments, including one done by Kim at the Korea Advanced Institute of Science and Technology in Daejeon, can’t quite be explained away just yet.

Balibar, for one, is still holding out a little hope for supersolidity. “I bet that in 10 years they discover it,” he says. “But it’s a risky bet.”