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Electron antics at magic angles.

The notion of a "metal" that contains no metal atoms may sound a little strange. But over the last few decades, researchers have synthesized a number of electrically conducting organic compounds. Curiously, several organic metals also become superconductors at sufficiently low temperatures. Now, scientists are finding that some of these substances have other unusual magnetic and electrical properties.

One of the most striking examples concerns the magnetic and electrical behavior of organic conductors based on a molecule called tetramethyltetraselenafulvalene (TMTSF). Stacked like pancakes, with each layer slightly offset, these negatively charged, planar molecules form an array with regularly spaced niches for small, positively charged ions. In the early 1980s, researchers were startled to discover. that these materials themselves become magnetic when subjected to a magnetic field.

"You simply turn on a magnetic field and you've got yourself a magnetic material; turn off the field and it's nonmagnetic," says physicist Michael J. Naughton of the State University of New York at Buffalo. In this case, the organic metal becomes an antiferromagnet, in which the spins of neighboring ions line up parallel to each other but in opposite directions.

In 1989, A.G. Lebed of the L.D. Landau Institute of Theoretical Physics in Moscow and Per Bak of the Brookhaven National Laboratory in Upton, N.Y, predicted that applying a strong magnetic field to a crystal of one of these materials would also cause dramatic decreases in its electrical conductivity at certain angles. Lebed dubbed these particular values "magic" angles. In essence, the theorists reasoned that electrons in the material - restricted by its crystal structure to motion in one dimension- would meet increased resistance when forced by the magnetic field to travel in certain directions.

Subsequent experiments by Naughton and his colleagues and by groups at Princeton University and in Japan found large changes in electrical conductivity at just the angles that Lebed and Bak had identified. But the researchers discovered that the electrical conductivity actually increases rather than decreases at those particular angles.

Theorists have weighed in with a number of explanations, but none of the theories appears completely satisfactory, "There has yet to be a consistent theory," Naughton says. "The issue isn't settled yet."

One promising approach, proposed by Princeton's Paul M. Chaikin, suggests that electrons traveling in the directions defined by Lebed's magic angles avoid what Chaikin calls "hot spots" - in some sense, places where electrons tend to be strongly deflected. In the presence of a magnetic field, electrons are typically swept into these hot spots. At magic angles, the coordinated motion of electrons in harmony with the organic metal's crystal lattice allows a fraction of the electrons to avoid the hot spots.

The Landau Institute's Victor M. Yakovenko argues that interactions between electrons, which allow them to coordinate their behavior, may play a crucial role. He suspects the same mechanism underlies the superconducting, antiferromagnetic, and electrical behavior of these organic metals.

But new experiments have added more puzzles. Researchers had already found that increasing the external magnetic field would magnify the conductivity peaks at the magic angles. Naughton's group went to magnetic fields higher than those previously used. "The effect does get stronger up to a certain field range, but by 30 teslas it is almost completely gone," Naughton says. "This is something new for the theorists to chew on." Naughton described his groups most recent findings at an American Physical Society meeting held this week in Seattle.
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Title Annotation:magnetic properties of organic conductors
Publication:Science News
Date:Mar 27, 1993
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