The first superconductors-materials that carry electricity without any resistance-were discovered in 1911. Half a century passed before physicists figured out how metals such as niobium perform that mind-bending feat at a few degrees above absolute zero. In 1986, researchers discovered complex compounds containing copper and oxygen that become superconductors at much higher "critical temperatures"-now as high as 138 kelvin. Twenty-four years later, such "high-temperature superconductivity" remains the biggest puzzle in condensed-matter physics.
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