Wednesday, March 18, 2009

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

A surprising and fascinating report coming out of the on-going APS March Meeting. A group from Berkeley is reporting the possible discovery of a supersolid in "... a gas of rubidium atoms..."!

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.


This appears to have a stronger and more definitive observation than the earlier report of possible supersolid state in solid He. It should be quite interesting to see if this gets verified, especially in having the ordered state to qualify as a "solid".


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