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'Hotter' Superconductors

Iron- and arsenic-based crystals form new class of materials

by Elizabeth K. Wilson
April 28, 2008 | A version of this story appeared in Volume 86, Issue 17

Credit: Adapted from Nature © 2008
Credit: Adapted from Nature © 2008

For the first time in years, a new class of high-temperature superconductors has been discovered, prompting labs around the world to begin synthesizing variants with ever higher superconducting temperatures.

Scientists have been ardently pursuing the development of high-temperature superconductors for decades. These materials have little or no electrical resistance at temperatures far warmer than the near-absolute-zero temperatures required for earlier generations of superconductors. Because higher temperatures are easier to maintain, the higher the operating temperature, the more practical the superconductor will generally be. That's why the potential of high-temperature materials has led to long technological wish lists with applications in electronics and other areas.

This newest family of high-temperature superconductors consists of layers of a fluorine-doped rare-earth metal oxide (lanthanum or samarium) sandwiched between layers of iron arsenide.

LaO(1-x)FxFeAs is the first set of these compounds, reported by physics professor Hideo Hosono and colleagues at the Tokyo Institute of Technology in March (J. Am. Chem. Soc. 2008, 130, 3296). The compounds have a superconducting transition temperature, or Tc, of 26 K. Although this Tc is high compared with that of ordinary superconductors, it pales next to those of copper-and- oxygen-based high-temperature superconductors discovered in the 1980s. Tc in that class can exceed 130 K.

Now, some of the original researchers have teamed up with Hiroki Takahashi and colleagues at Nihon University in Tokyo and have found that putting the new compounds under pressure bumps up the Tc of the same class of materials to 43 K (Nature, DOI: 10.1038/nature06972). Takahashi says he believes the pressure causes an increase in charge-carrier concentration in the FeAs layer, leading to the higher-temperature superconductive behavior.

Douglas Scalapino, a theoretical physics professor at the University of California, Santa Barbara, who studies superconductivity, says the discovery is "very exciting," noting that many different atomic substitutions can be made to create variants of the same class of materials.

In the meantime, several other labs in China have also reported synthesizing similar compounds with even higher Tc. A team led by Zhong-Xian Zhao at the Institute of Physics of the Chinese Academy of Sciences in Beijing just reported that SmO(1-x)FxFeAs superconducts at 55 K. The results aren't yet published in a peer-reviewed journal but are posted on the preprint archive

Scientists are still puzzling over the detailed mechanism of high-temperature superconductivity, but it appears to involve an electron pairing process that allows the electrons to sail unimpeded through layers of the crystal lattice. The advent of the new materials, Scalapino says, gives theorists a "second chance to understand the pairing mechanism that I believe underlies the whole range of these materials."



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