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Exotic state of matter shows up in ultracold gas of rubidium atoms: supersolid acts like a solid and superfluid simultaneously.

Hallmarks of an exotic, predicted state of matter called a supersolid have been spotted in a gas of ultracold rubidium atoms. In the same matter, researchers found signs of the seemingly disparate properties of both solidity and superfluidity, the frictionless flow of atoms.

DanStamper-Kurndescribed on March 18 two telltale signs suggesting that this weird matter may indeed be a supersolid. The new matter is "a gas, which is superfluid, and also shares properties of a solid," said Stamper-Kurn, of the University of California, Berkeley. If confirmed, a rubidium supersolid could help scientists better understand the properties of this strange state of matter.

"What we've seen is an ability to describe a peculiar state of nature," comments Paul Grant, a former visiting scholar at Stanford University and an IBM research staff member emeritus. If the researchers can extend their "interesting basic physics" to come up with new ideas and applications, Grant says, "there may be a Nobel Prize there."

A supersolid is defined by two seemingly contradictory properties. The atoms inside it are arranged in a crystalline, regular pattern, like any solid. At the same time, the atoms flow through the supersolid in an unrestricted way.

Although theorists predicted the existence of a supersolid more than three decades ago, the first observation came in 2004 when researchers at Pennsylvania State University found evidence for supersolid behavior by helium. The interpretations of those experiments are still being debated. Some researchers proposed that the impurities in the helium crystal, and not an intrinsic property, may cause the superfluid behavior.

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Stamper-Kurn and colleagues used magnets and lasers to trap rubidium atoms in a surfboard-shaped area. The team used the lasers to cool the atoms to temperatures around 500 nanokelvins, just above absolute zero.

At those ultracold temperatures, the rubidium gas exhibits a solid, crystalline structure, the team reports online at arxiv.org/abs/0901.3800. "There's some kind of distinct checkerboard pattern of magnetization that spontaneously forms," said Stamper-Kurn. Slight magnetism of each atom, called a dipole moment, may form the pattern, he said.

The researchers next checked for signs of superfluidity, measuring the rubidium crystal to see whether the atoms moved in a concerted way, called coherence. Using a technique called atom interferometry, the team found a zebra-striped interference pattern that could occur only if the atoms were in lock-step. This pattern was present over long ranges of the rubidium atoms.

"The coherence is required for superfluid flow. It provides the hallmarks of superfluidity," says Charles Clark of the National Institute of Standards and Technology's Joint Quantum Institute based at the University of Maryland in College Park.

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Title Annotation:Physics
Author:Sanders, Laura
Publication:Science News
Article Type:Report
Geographic Code:1USA
Date:Apr 11, 2009
Words:436
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