Ultracold plutonium compound shows no resistance. (Cold War Conductor).

Researchers studying the crystalline structure of radioactive plutonium have happened onto the first plutonium-based superconductor. Like other superconductors, this one carries electricity with zero resistance, but it doesn't fit neatly into any known family of superconducting substances.

Plutonium, the explosive heart of most nuclear weapons, is too radioactive and toxic for the find to lead to any practical applications. But the puzzling new alloy is opening a route to studying some poorly understood aspects of superconductivity, says John L. Sarrao of Los Alamos (N.M.) National Laboratory, who led the experiment.

The path to the new superconductor traces back to an obscure scientific question from the Manhattan Project. Scientists making the first nuclear weapons found that when they heated plutonium, they could transform its brittle, room-temperature form into a ductile, so-called delta phase that was more easily machined into weapons parts. Moreover, by adding a little gallium to the hot material, they could get this favorable structure to persist even after the alloy cooled to room temperature. However, no one ever figured out gallium's action.

To investigate this puzzle, Sarrao and his Los Alamos coworkers created a new material that's structurally related to delta-phase plutonium. They blended and heated plutonium with gallium and cobalt and then slowly cooled the molten mixture. To the scientists' surprise, tests showed that the resulting compound is a superconductor at cryogenic temperatures below 18.5 kelvins.

A material becomes a superconductor when its free-roaming electrons, which ordinarily repel each other, form pairs that can zip through the material's crystal lattice unimpeded (SN: 9/7/02, p. 158). In conventional superconductors, atomic vibrations induce the electron pairing. Most of these materials need to be chilled below 20 K before they shed electrical resistance.

In a class of superconductors that's less well understood, the electron-pairing mechanism remains obscure. Some copper-oxide-based members of this class retain their superconductivity at temperatures as high as 160 K, about as warm as the coldest terrestrial temperature ever recorded outside a laboratory. Other members lose their electrical resistance only within a couple degrees of absolute zero. The new material resembles these in structure and some magnetic and electrical properties, Sarrao's team reports.

The new plutonium-gallium-cobalt material could be a missing link within this second class of superconductors because it loses electrical resistance at an intermediate temperature, Sarrao says. If further studies of the new compound's properties confirm it as a member of this class, they may also shed light on the electron-pairing mechanism that renders these materials superconductive, he adds.

Sarrao and his colleagues from Los Alamos, the University of Florida in Gainesville, and the European Commission's Institute for Transuranium Elements in Karlsruhe, Germany, describe the new superconductor in the Nov. 21 Nature.