In The Field

APS: When is a solid not a solid? When it’s a supersolid.

supersolid.JPGposted on behalf of Ed Gerstner

One of the advantages of being just one editor in an editorial team is that when something comes along that you just don’t get, you can always try to pass responsibility for handling submissions on that topic to someone else. For me, supersolids was one of those things. They’re fluids, superfluids in fact, but they have characteristics of a solid. What?!? Try as I might, I couldn’t get my head around it.

But when I picked up a buzz about new evidence of supersolidity in an entirely new system, I figured it was time to push through this particular mental block.

The new results come from Dan Stamper-Kurn at UC Berkeley, who works on Bose-Einstein condensates (BEC) of ultracold gases. The atoms in a Bose-Einstein condensate don’t exist at well defined positions like the atoms in a solid crystal. Each and every atom in a BEC spreads itself over the entire volume of the condensate, which can be tens of micrometres wide and hundreds of micrometres long. In the profoundest sense, a BEC has no atomic structure — not even the amorphous structure of the atoms that jostle about in a conventional liquid. This is one of the things that puts the ‘super’ in a superfluid.

And yet when Stamper-Kurn’s group look at variations in the magnetic polarization of a trapped ultracold quantum gas of rubidium atoms (by shining light through it), they see sees a handful of microscopic blobs that order themselves around each other like the beginnings of a crystal (see the figure 2 of their preprint). It’s not a typical crystal of course, the blobs are not atoms or groups of atoms forming in localized clusters in the gas — the density of the gas remains smooth and homogeneous. But its magnetic behaviour makes it look like a crystal. And crystalization is the signature of a solid.

And it’s as simple as that. A supersolid is a thing that has the properties of both a superfluid — in this case a quantum gas that has no material structure — and a solid — in this case crystalline order amongst its magnetic domains.

What does this all mean? I’ll tell you. I don’t know. But finally I can see what it looks like.

Ed Gerstner

image: arXiv:0901.3800v1

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