A year ago, physicists discovered a new family of high-temperature superconductors: materials that carry electricity without resistance at temperatures as yet inexplicably far above absolute zero. Immediately, researchers wondered whether the compounds, which contain planes of iron and arsenic atoms, work in the same way as the only other high-temperature superconductors, the copper-and-oxygen, or "cuprate," compounds that have defied explanation since their discovery in 1986 (Science, 16 May 2008, p. 870). New data suggest that a key step in making all the superconductors-"doping" them with impurities-plays a very different role in each of the two families.
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