Evidence mounts for an exotic supersolid

Rubidium atoms simultaneously act like a solid and a superfluid

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

Reporting March 18 at a meeting of the American Physical Society, Dan Stamper-Kurn described two telltale signs that suggest this weird state of 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 IBM research staff member emeritus. If the researchers are able to extend their “interesting basic physics” results 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, but at the same time, the atoms are able to flow through the supersolid in an unrestricted way.

Although theorists first predicted the existence of a supersolid over three decades ago, the first observation came in 2004 when researchers at Pennsylvania State University found evidence for supersolid behavior in 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 of the matter, may be causing the superfluid behavior detected in helium.

Stamper-Kurn and his colleagues used magnets and lasers to trap rubidium atoms — which have a slight positive and negative magnetic charge at opposite ends — in a surfboard-shaped area. The researchers then used lasers to cool the atoms to temperatures hovering around 500 nanokelvins, just above absolute zero.

At those ultracold temperatures, the rubidium gas exhibits a solid, crystalline structure. “We found to our amazement that when we cooled this rubidium gas to very cold temperatures, it crystallized,” says Stamper-Kurn. “There’s some kind of distinct checkerboard pattern of magnetization that spontaneously forms. It’s no longer homogeneous fluid.” The slight magnetism of the rubidium atoms, called a dipole moment, may form the pattern, he says.

The observation of solid properties in a gas initially gave the researchers pause. “As an experimentalist, the first time you see something remarkable, you’re certain that you’ve made a mistake.” Repeating the experiments, however, ruled out many other possible explanations for the results, which are presented in a paper online at arxiv.org/abs/0901.3800.

The researchers next checked for signs of superfluidity, measuring the rubidium crystal to see if the atoms moved in a concerted way, called coherence. An ocean wave that travels to the shore in one unified line exhibits coherence.

To look for this cohesive behavior, Stamper-Kurn’s team used a technique called atom interferometry. First, the scientists hit two separate groups of rubidium atoms in the material with a beam of light. Then, as the two groups of atoms bounced off the two light beams, the atoms interfered with each other to produce a zebra-striped interference pattern, which could occur only if the atoms were in lock-step, exhibiting coherence. This pattern is present over long ranges of the rubidium atoms, a direct indication of superfluidity. Those results, not yet published, and the observation of a crystalline structure together strongly suggest that the material created by Stamper-Kurn and his team is a supersolid.

“The coherence is required for superfluid flow. It provides the hallmarks of superfluidity,” says Charles Clark of the Joint Quantum Institute at the National Institute of Standards and Technology in Gaithersburg, Md. In January, Clark and colleagues published results in Physical Review A predicting several key hallmarks of a simplified one-dimensional rubidium supersolid. The theoretical work predicts how to “create an object that has the internal structure of a crystal but with the overall coherent flow of a matter wave,” he says. Stamper-Kurn’s results may be just that.

Stamper-Kurn says he is amazed and still perplexed by the dual nature of the rubidium material. “How can it be solid and show this highly patterned state, and at the same time, have this amazing uniformity? That would seem to me contrary to the possibility of becoming a superfluid.”

Rubidium atoms simultaneously act like a solid and a superfluid
CRYSTAL GAS Trapped in a surfboard-shaped formation, ultracold rubidium atoms self-organize into a regular crystal pattern (seen more clearly at right), one of the hallmarks of supersolidity. IMAGE CREDITS: Mukund Vengalattore and Dan Stamper-Kurn

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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