Europium’s superconductivity demonstrated

The rare earth metal is the 53rd naturally occurring element to possess the property

An old element just learned a new trick under pressure. When cooled and squeezed very hard, the soft metallic element europium turns into a superconductor, allowing electrons to flow unfettered by resistance, a study appearing May 13 in Physical Review Letters shows. The results make europium the 53rd of the 92 naturally occurring elements to possess superconductivity, which, if harnessed, could make for more efficient energy transfer.

53RD SUPERCONDUCTOR Researchers used a diamond anvil cell like the one shown to exert a large pressure on europium, causing the metal to become a superconductor. This view shows one of two opposing 16-faceted diamond anvils in the middle, surrounded by a coil that detects superconductivity. Debessai, et al., Physical Review B, 2008.

Europium, a rare earth metal with a silver color, is strongly magnetic at everyday temperatures and pressure. Study coauthor James Schilling of Washington University in St. Louis suspected europium would superconduct if researchers could overcome its magnetism, which disrupts a certain type of electron pairing that superconductivity requires. “Most of the rare earths would be superconducting at ambient pressure, except that they’re magnetic,” Schilling says.

Europium’s magnetism stems from the electrons in its 4f subshell, or orbital. Typically, europium atoms have seven electrons in this orbital. Schilling and his colleagues found that under huge amounts of pressure, though, one of these electrons jumps out of this shell and renders europium nonmagnetic. “If you can force one of the seven electrons out, it so happens that the ground state is nonmagnetic, and that opens the door for it to become superconducting,” Schilling says.

To achieve the extreme conditions necessary to squeeze europium’s seventh electron out of its shell, the researchers put the metal in a special device called a diamond anvil cell — a miniature torture device designed to exert extremely high pressures by squeezing the sample between two large diamonds. The researchers then cooled europium down to about 1.8 kelvins (–271.35º Celsius), a frigid temperature near absolute zero.

At pressures around 80 gigapascals, or about 800,000 times the pressure exerted by the atmosphere at sea level, europium lost its magnetism. Electrons could flow freely through the metal without resistance.

“This is very difficult experimentally to do,” says Jeffrey Lynn, a physicist at the National Institute of Standards and Technology in Gaithersburg, Md. “And that’s interesting from a scientific standpoint.”

The extreme conditions required precludes europium from being a useful superconductor in everyday life, says Lynn. “It’s really more for understanding what’s going on than for any practical applications.”

Some unconventional superconductors made from materials like copper and iron offer more promise for storing and transmitting energy, since they may one day operate at higher temperatures and lower pressures than superconductors like europium.

Still, Schilling says, “Superconductivity is an area where it’s very difficult theoretically to have the last word, to really know what’s possible and what’s not…. Anything one can do to further the understanding of superconductivity might eventually help one design a better superconductor.”

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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