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Experiment Reveals Conductor-insulator Transition

University of Chicago physicists have, for the first time, directly observed a group of atoms transform from a conducting state into an insulating state.

“It opens up much richer phenomena to explore,” said Cheng Chin, an assistant professor in physics at the University. Chin’s team conducted the experiment as the first step in a project to simulate the dynamics of electrons in a solid.

“How you can make the transition from a conducting material to a non-conducting material is difficult to conceive,” said Chin. But his team actually observed such a transition using super-cooled atoms to simulate the behavior of electrons.

“It’s nearly impossible to resolve the dynamics of electrons,” Chin said, because they move from atom to atom in trillionths of a second. The Chicago physicists dodged this problem by cooling a single layer of cesium atoms to temperatures near absolute zero (minus 459.67 degrees Fahrenheit). Then they magnetically controlled the motion of the atoms on a millisecond time scale (thousands of a second). This is a billion times slower than electrons move, but the physics remains the same.

“We made a thin film of atoms, and then we watched how they distributed themselves inside our chamber.”

What they observed confirmed a prediction that another team of scientists made in 2000: While the atoms are in a superfluid state (conducting), they experience very little repulsive force between each other. When moving freely, these atoms can become compressed with the application of pressure.

“There’s a certain mobility when you apply a force. You can easily compress a conducting sample,” Chin said.

But when the Chicago researchers applied a magnetic field, initiating a much greater repulsive force between the atoms, they became jammed and could not be deformed. The atoms had entered an incompressible insulating state.

Citation: Gemelke, Nathan; Zhang, Xibo; Hung, Chen-Lung; and Chin, Cheng, “In-situ Observation of Incompressible Mott-Insulating Domains of Ultracold Atomic Gases,” Nature, Aug. 20, 2009.

Funding sources: National Science Foundation, Defense Advanced Research Projects Agency, and the Grainger Foundation.

Steve Koppes | Newswise Science News
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