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Signs of a supersolid at the March APS meeting

PITTSBURGH—Look in that lab: it's a gas, it's a solid, it's a superfluid—it's SuperSolid! Well, maybe.

The "it" in question is a collection of rubidium atoms cooled to within a whisker of absolute zero and the lab is physicist Dan Stamper-Kurn's at the University of California, Berkeley. His group is working on clouds of the ultracold atoms that exhibit properties of multiple states of matter at once. Stamper-Kurn announced the details of the research yesterday at the American Physical Society meeting here.

Ultracold rubidium has achieved fame before, being one of the gases first turned into a Bose-Einstein condensate in the mid-1990s (at that time Stamper-Kurn was a graduate student in 2001 Nobelist Wolfgang Ketterle's group at the Massachusetts Institute of Technology, but that is another story, involving rubidium's cousin sodium).

In a Bose-Einstein condensate, or BEC, a swarm of atoms all plunge into the same quantum state and take on exotic characteristics such as superfluidity, the ability to flow in perfect unison without any of the usual slowing effects of atoms jostling against atoms or, say, ketchup molecules against ketchup molecules. That extreme slipperiness makes a superfluid sound like the complete antithesis of a solid, yet the idea of a material with both properties is taken seriously by quantum physicists.

Indeed, in 2004 Moses Chan and Eun-Seong Kim of Pennsylvania State University reported evidence of solid helium having a small superfluid fraction. Subsequent work, though, has left the status of supersolid helium unclear.

The solid helium in Chan and Kim's work involved the kind of solidity that anyone might recognize. The solid in Stamper-Kurn's experiment is a much more delicate substance.

Each of his group's rubidium atoms has an associated magnetic moment—much like a tiny compass needle. In a lot of ultracold atomic research, magnetic fields hold the clouds of atoms in place, but those setups result in the "compass needles" all lining up. The Berkeley group instead used a laser beam to trap their atoms. The form of the trapping beam results in an atom cloud shaped much like a surfboard, but more important, the absence of an applied magnetic field leaves the magnetic moments free to point any which way.

Stamper-Kurn's team found that when the gas was cold enough, the magnetic moments spontaneously lined up in approximate patterns, forming what is called a magnetic crystal. Images of the gas colored to represent the different orientations of the magnetic moments at each location show regular patterns of spots, reminiscent of lattices of atoms in a crystal. Imagine the surfboard spattered with paintball shots that look random, but on closer examination of just the red spatters, say, a rough regular pattern emerges. In everyday terms, the atom cloud is about as solid as a smoke ring, but the crystalline pattern of the magnetism is what counts for physicists.

The researchers demonstrated the "super" part of the story by hitting their atoms with the atomic physicists' weapon of choice, laser pulses (not paintballs). The pulses knocked out two subgroups of the atoms, like two ghostly copies of the main surfboard. When the two ghost clouds merged, a striped pattern appeared, the standard signal of interference, which in turn indicated that the atoms were marching along in the kind of lockstep known as quantum coherence—the same feature that makes laser beams so useful and that is fundamental to BECs, superfluids and superconductors.

Stamper-Kurn is careful not to claim discovery of the first (or maybe second) supersolid. His group has to complete further studies of the atom clouds and better understand just what is happening in the system, and he's the first to point out the clouds may not be a true supersolid at all. But whether the supersolid rubidium ultimately pans out or not, he's having the time of his life investigating these questions.

Photo of Stamper-Kurn courtesy of the University of California, Berkeley

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