Super-thin supersolid points to superglass

By Hamish Johnston

After a buzz of activity in 2004-2008, it looked like it was going to be a quiet year for supersolid enthusiasts — until about two weeks ago, when two papers appeared in Science on that mysterious state of matter that may (or may not) exist.

One of the papers (by Seamus Davis and crew at Cornell) suggested that supersolids may in fact be “superglasses”.

Now, supersolid pioneer Moses Chan and colleagues at the Pennsylvania State University have published a paper in Physical Review Letters that seems to back-up the superglass theory.

The first evidence for a supersolid was seen in 2004 by Chan and Eun-Seong Kim, who noticed that around 1% of the atoms in a sample of solid helium seemed to “decouple” and flow effortlessly through the rest of the mass like a superfluid.

While this effect has been reproduced in several other labs — the relative size of the decoupling has varied from 0.02% to 20% in different samples.

One common thread through these experiments seems to be the amount of disorder in the solid helium — with nearly-perfect crystals showing relatively small amounts of supersolidity and disordered solids showing lots.

Another thing is that the solids with the highest decoupling are very thin. Indeed, the 20% figure was seen by John Reppy and Sophie Rittner at Cornell in a film just 150 μm thick. This seemed to suggest that there is an ideal surface-to-volume ratio for supersolidity.

In their latest work, Chan and team also looked at a 150 μm thick film of solid helium — and saw a decoupling of about 1%.

So what was different?

Because of their experimental set-up, Reppy and Rittner froze their helium in about one minute, whereas Chan’s samples took at least 4 hours to solidify. Chan believes that the rapid cooling of the Reppy/Rittner film could leave it in a (highly disordered) glassy state , which seems to boost supersolidity.

Things are heating up again for supersolids!