Is It a Gas, Fluid, Solid, or All of the Above?

Solid evidence? An ultracold gas of rubidium atoms shows a crystalline-like arrangement of magnetic regions, making a possible supersolid material.

M. Vengalattore et al., (24 January 2009)

PITTSBURGH, PENNSYLVANIA--Five years ago, researchers in the United States saw the first evidence of a "supersolid," a bizarre state of matter in which crystals of ultracold helium could flow like a liquid without viscosity. But the evidence for supersolidity in helium has not been ironclad. Now, there is a new contender for the supersolidity claim. At the American Physical Society meeting here today, Dan Stamper-Kurn, a physicist at the University of California, Berkeley, reported evidence that a gas of rubidium atoms might form a supersolid. If the new observations hold up, they could usher in a new class of materials ideal for understanding the quantum behavior of matter.

To qualify as a supersolid, a collection of atoms must pass two tests. First, it must have a regular arrangement, like the alternating sodium and chlorine atoms in a grain of table salt. Second--and this is the weird part--the atoms must all adopt the same quantum-mechanical state. The second property, called coherence, makes it possible for researchers to see their wavelike quantum nature. Previous teams spotted it when they created Bose-Einstein condensates in a gas of rubidium atoms back in 1995. But the atoms in those gases aren't ordered, so they didn't make up a supersolid. Because rubidium is magnetic, however, Stamper-Kurn and his Berkeley colleagues thought that the magnetic interactions between rubidium atoms in the gas might nudge them to adopt a type of regular spacing like atoms in a solid.

To look for this ordering, Stamper-Kurn's team used a conventional laser trapping technique to confine a gas of millions of rubidium atoms in an oblong, surfboardlike trap. They then cooled the sample to below 500 nanokelvin. Lastly, they hit their collection of rubidium atoms with a beam of circularly polarized light, which is reflected differently by atoms with a different magnetic orientation and can, therefore, reveal the magnetic orientation of the atoms in the sample. What they saw was that within their optical trap, the rubidium atoms ordered themselves into an array of 5-micrometer-square domains, inside which all of the atoms adopted a similar magnetic orientation. What's more, these domains adopted a crystalline-like ordering, with alternating domains with different magnetic directions. This ordering wasn't perfect like the regular lattice of sodium and chlorine atoms in table salt. But it's not random either (see picture). "There is some emergent order which shows up in this system," Stamper-Kurn says.

Once the Berkeley researchers spotted the ordered makeup of the atoms, they decided to check whether the gas was coherent as well. Using another laser, they nudged two groups of rubidium atoms already in their trap. They found that the atoms interfered with each other in the same way that two coherent light beams create an interference pattern of light and dark stripes, an unmistakable sign of their wavelike quantum nature.

The new work is "fabulous," says Charles Clark, a physicist at the National Institute of Standards and Technology in Gaithersburg, Maryland. Unlike the bulk-sized supersolid helium, ultracold gases can be easily manipulated, Clark notes. That could open the door to new experiments testing the behavior of supersolids, he says. But Stamper-Kurn cautions that the case for supersolidity in rubidium isn't closed yet. For one thing, the crystalline-like magnetic ordering isn't perfect. And the laser trap's surfboard shape allows scientists to see only five or six magnetic domains across its width, although they can see dozens along its length. Many researchers will likely want more proof that the gas's ordering extends further, before crowning it a supersolid.

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