PHYSICISTS DISCOVERED A SECOND FAMILY OF HIGH-TEMPERATURE superconductors, materials that carry electricity without resistance at temperatures inexplicably far above absolute zero. The advance deepened the biggest mystery in condensed-matter physics. In February, a group in Japan reported the first material, fluorinedoped lanthanum iron arsenic oxide (LaFeAsO (1-x) F x ), which is superconducting up to a “critical temperature” of 26 kelvin. Within 3 months, four groups in China had replaced the lanthanum with elements such as praseodymium and samarium and driven the temperature for resistancefree flow up to 55 kelvin. Others have since found compounds with different crystal structures and have bumped the critical temperature up to 56 kelvin. For a critical temperature, that’s not so hot. The record is 138 kelvin for members of the other family of high-temperature superconductors, the copper-and-oxygen, or “cuprate,” compounds discovered in 1986. Still, the iron-based materials have created a stir, in part because they might help solve the enduring mystery of how the cuprates work. The $64,000 question is whether the two families work the same way. So far, evidence points in both directions.