New Research: Against All Odds, Plutonium Is Latest Superconductor

Scientists have discovered superconductivity in a most unlikely place: the highly radioactive element used to make nuclear weapons. In an article set to appear Thursday 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. Superconductors conduct electricity without any resistance. Discovered in the early 20th century, the first ones functioned only at extremely cold temperatures of a few degrees Kelvin, or about 450 degrees below zero Fahrenheit. In 1986, however, two Swiss researchers created a copper- containing ceramic that became a superconductor at what was considered an astonishingly high temperature of 35 degrees Kelvin, about minus 400 F. The discovery touched off a flurry of interest and activity because superconductors that could operate within range of room temperature would have enormous practical value. For example, electric trains driven by superconducting magnets would levitate slightly above their tracks, operating virtually without friction and so requiring little electricity to run. Superconducting power lines would transmit electricity from power plants to homes without most of the energy loss that occurs now, while superconducting generators would lose little heat energy and so could produce electricity less expensively than current generators. The impediment to such advances is the low temperatures required for superconductors to work. Although scientists working in the wake of the 1986 discovery now have created superconductors at temperatures as high as 138 Kelvin, or about minus 200 F, that remains too low for many of the most powerful potential uses, because cooling the materials requires as much or more energy than is gained by their being superconductors. Exactly how and why superconductivity works, meanwhile, remains something of an open book. The classic explanation of the phenomenon, first advanced in 1957 by three American physicists, does not adequately explain high- temperature superconductors of the sort discovered in 1986 and more recent years, Stewart said. The plutonium-based superconductor is another discovery that seems to go against the traditional theory, he said. 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 superconducted while they were measuring the 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. Stewart worked on a team, led by Los Alamos physicist Luis Morales, that helped establish the material’s superconductivity by measuring a quality called specific heat. The measurement, one of a couple tools used to confirm the plutonium's superconductivity, is not an easy one to make, Sarrao said. "The heat that results from radioactive decay makes heat capacity measurements very difficult, in part because it's difficult to determine the actual temperature of the sample," Sarrao said. "Dr. Stewart made important contributions to the heat capacity measurements." 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." Correspondence and requests for materials should be addressed to J.L.S. (e-mail: sarrao@lanl.gov).