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See Inside June 2008

Iron Exposed as High-Temperature Superconductor

New class of superconductor may help pin down mysterious physics



David Parker IMI/University of Birmingham High TC Consortium/Photo Researchers, Inc.

For more than 20 years, the only known superconductors that worked far above liquid-helium temperatures were a few dozen compounds—virtually all based on copper. Now scientists have discovered the first high-temperature superconductors based on iron. These novel materials could help unravel one of the biggest mysteries in science—how exactly the high-temperature versions work.

In superconductors electric current flows completely without resistance. For decades, the phenomenon was thought to occur only near absolute zero. The cold tames the vibrations of the atoms making up the substance in such a way that electrons can overcome their natural repulsion for one another. The altered vibrations, called phonons, cause the electrons to pair up; so coupled, they can then move freely through the atomic lattice.

Starting in 1986, however, physicists began discovering a new class of superconductors operating well above absolute zero, to temperatures as high as 160 kelvins (–113 degrees Celsius). These materials, dubbed cuprates, typically consist of copper oxide layers sandwiched between other substances. The structure of the cuprates and the high temperature interfere with the mechanisms that drive conventional superconductors, leading physicists to try to come up with new explanations.

A serendipitous discovery is now forcing investigators to expand their ideas on superconductivity. Materials scientist Hideo Hosono of the Tokyo Institute of Technology and his colleagues were looking to improve the performance of transparent oxide semiconductors but ended up discovering the first iron-based, high-temperature superconductor.

The crystalline material, known chemically as LaOFeAs, stacks iron and arsenic layers, where the electrons flow, between planes of lanthanum and oxygen. Replacing up to 11 percent of the oxygen with fluorine improved the compound—it became superconductive at 26 kelvins, the team reports in the March 19 Journal of the American Chemical Society. Subsequent research from other groups suggests that replacing the lanthanum in LaOFeAs with other rare earth elements such as cerium, samarium, neodymium and praseodymium leads to superconductors that work at 52 kelvins.

High-temperature superconductivity in these layered iron compounds completely surprised investigators, who thought that the magnetic nature of iron would disrupt the pairing of electrons. Perhaps, as seems to be the case for cuprates, the electrons get paired with the aid of spin fluctuations—disturbances in the magnetic fields of atoms making up the superconductor. “These ironbased superconductors could give us new hints on how to understand cuprates,” says physicist Kristjan Haule of Rutgers University.

On the other hand, the spin fluctuations that could glue together cuprate electrons might not be enough for those in the iron-based materials. Instead orbital fluctuations—or variations in the location of electrons around atoms—might also prove crucial, Haule speculates. In essence, the iron-based materials give more freedom to electrons than cuprates do when it comes to how electrons circle around atoms.

Orbital fluctuations might play important roles in other unconventional superconductors as well, such as ones based on uranium or cobalt, which operate closer to absolute zero, Haule conjectures. Because the iron-based superconductors work at higher temperatures, such fluctuations may be easier to research.

Besides illuminating the theoretical underpinnings of superconductivity, the discovery “makes us ask if there are other high-temperature superconductors we haven’t found yet in unexpected places and if there are even higher temperatures these can work at,” remarks theoretical physicist David Pines of the University of California, Davis, who is also founding director of the Institute for Complex Adaptive Matter. In trying to boost the critical temperature, experiments should focus not only on swapping in other elements but also on layering the compounds. That should improve them just as it does for cuprate superconductors, Haule thinks.

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