Courtesy:  University of Florida, et al
November 20, 2002

GAINESVILLE, Fla. --- Scientists have discovered superconductivity in a most unlikely place: the highly radioactive element used to make nuclear weapons.

In an article in the journal Nature, a group of researchers, including a University of Florida physicist, report discovering a plutonium-based electrical superconductor. The finding is significant because plutonium, the active ingredient in atomic bombs, has physical properties that should prevent it from behaving as a superconductor - suggesting current theories about this phenomenon may not apply to this element.

“This is anomalous superconductivity, which is fascinating,” said Gregory Stewart, a UF professor of physics and contributing author to the paper.

Stewart said John Sarrao, the lead author on the Nature paper, and his colleagues at the Los Alamos National Laboratory in New Mexico discovered the plutonium compound PuCoGa5 superconducted while they were measuring its magnetic behavior. To their surprise, a probe of the material’s magnetic properties revealed diamagnetic, or “anti-magnetic” behavior, a telltale indicator of superconductivity, he said. That was unexpected because plutonium, a heavy element in the actinide group, very often forms compounds that are highly magnetic; never before had a compound containing plutonium been found to be superconducting.

“It would be like finding an excellent material for building skyscrapers from a new recipe for Jell-O,” Stewart said. “You just wouldn’t expect it.”

More surprising still, the plutonium did not begin superconducting at 1 or 2 degrees Kelvin, which one might expect for a material that was not very superconductive. Instead, it began superconducting at 18 Kelvin, or about minus 427 degrees F, which prior to 1986 would have been considered a high-temperature superconductor, Stewart said.

The discovery has no immediate practical value but is important because it adds a new dimension to the study of superconductivity, Stewart said. “You can’t make practical materials out of something as radioactive and chemically poisonous as plutonium," he said, "but John Sarrao and this collaborative team have made a big leap in understanding superconductivity from a fundamental point of view.”

The team hopes that future research will unearth superconductivity in other transuranic compounds with lower toxicity.

Note: According to Dr. Stewart "the Karlsruhe and Los Alamos crystallographers have decided that [the lattice of this compound] is, in fact, tetragonal (as are the other 115's like CeIrIn5 and CeCoIn5). a=4.232 A and c=6.686 A."

Note 2: While the prototype for this structure is HoCoGa5, Americium is normally the first element in the actinide series that behaves like a rare earth.

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