Kamihara et al., JACS, 130 (2/23/08)

Between the sheets.
In new superconductors, electrons flow through layers of iron and arsenic interspersed among layers of other atoms.

Second Family of High-Temperature Superconductors Discovered

Staff Writer

Researchers in Japan and China have discovered a new family of high-temperature superconductors--materials that conduct electricity without any resistance at inexplicably high temperatures. Physicists around the world are hailing the discovery of the new iron-and-arsenic compounds 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.

"It's possible that these materials will provide a cleaner system to work with, and suddenly [the physics of] the cuprates will become clearer," says Hai-Hu Wen, a physicist at the Institute of Physics (IoP) at the Chinese Academy of Sciences in Beijing. But Philip Anderson, a theorist at Princeton University and a Nobel Laureate, says that the new superconductors will be more important if they don't work like the old one. "If it's really a new mechanism, God knows where it will go," he says.

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. On 23 February, Hideo Hosono of the Tokyo Institute of Technology and colleagues reported in the Journal of the American Chemical Society that lanthanum oxygen fluorine iron arsenide (LaO1-xFxFeAs) becomes a superconductor at 26 kelvin.

Then Chinese researchers took over. On 25 March, X.H. Chen of the University of Science and Technology of China in Hefei reported that samarium oxygen fluorine iron arsenide (SmO1-xFxFeAs) goes superconducting at 43 kelvin. Three days later, Zhong-Xian Zhao of the IoP reported that praseodymium oxygen fluorine iron arsenide (PrO1-xFxFeAs) has a "critical temperature" of 52 kelvin. On 13 April, Zhao and his team showed that the samarium compound becomes a superconductor at 55 kelvin if it is grown under pressure. All the materials have the same crystal structure, and calculations suggest that vibrations simply do not provide enough pull to account for such high critical temperatures.

At least four different groups in China, including three at IoP, have synthesized new compounds and posted results on the arXiv.org preprint server (www.arxiv.org).

The first question on everyone's mind is whether the new high-temperature superconductors work the same way as the old ones. Anderson says they cannot because the older materials evolve from a state with one electron per copper ion, whereas new materials evolve from a state with two electrons per iron ion. But Steven Kivelson, a theorist at Stanford University in Palo Alto, California, notes that the old and new materials both have planar structures, start off as bad conductors, and exhibit a type of magnetism known as antiferromagnetism. "That's enough similarities that it's a good working hypothesis that they're parts of the same thing," he says.

All agree that the new materials will generate intense interest and that the next step is to synthesize higher quality samples consisting of a single pristine crystal.

Posted in Asia, Technology, Physics