Nanomaterials

Novel Iron-based Superconductors Discovered

16 April 2008

credit: JACS

You may not see room temperature superconductors in the near future, but scientists are working hard toward this goal. Recent discovery of Fe-based superconductors by Japanese scientists set scientific community a-buzzing.

This Februry, researchers at the Tokyo Institute of Technology reported a new superconductor based on iron and arsenic in JACS (v 130, p 3296-3297, 2008). Physicists around the world are hailing the discovery as a major advance, as the only other high-temperature superconductors are the copper-and-oxygen compounds, or cuprates, that were discovered in 1986. Those older materials netted a Nobel and ignited a firestorm of research, but physicists still don't agree about how they work, leaving high-temperature superconductivity the biggest mystery in condensed matter physics. Some researchers hope the new materials will help solve it.

Superconductivity is nature's best parlor trick. Ordinarily, electrons flowing in a metal lose energy as they ricochet off defects in crystalline material. In superconductors, the electrons experience no such drag and just keep going. That's because below a certain temperature, they form pairs. Deflecting an electron then requires breaking the pair, and at low temperatures there isn't enough energy around to do that. So the duo waltzes along unimpeded.

In an ordinary superconductor, the pairs are held together by vibrations rippling through the material's framework of positively charged ions. Most physicists, however, think that mechanism cannot explain the cuprates, which work at temperatures as high as 138 kelvin. In them, each compound contains planes of oxygen and copper ions arranged in a square pattern. Electrons hop from copper ion to copper ion and somehow pair, although physicists do not agree about how that happens.

The new materials resemble the cuprates in some striking ways. They are also layered materials, but instead of copper and oxygen, they contain planes of iron and arsenic along which the electrons presumably glide. Between the planes lie elements such as lanthanum, cerium, or samarium mixed with oxygen and fluorine.

The first question on everyone's mind is whether the new high-temperature superconductors work the same way as the old ones. One thing is for sure: the new materials will generate intense interest and that the next step is to synthesize higher quality samples consisting of a single pristine crystal.