Throughout the history of supercon-I ductivity, optical spectroscopy - through the scattering of light by a material - has been a vital tool. It was the existence of a gap in the excitation-energy spectrum of electrons, first observed in optical studies, that set Bardeen on the path to the celebrated Bardeen-Cooper-Schrieffer theory of superconductivity. That theory, in which electrons move as Cooper pairs, is now the established description of the low-temperature phenomenon. On page 714 of this issue, Hwang et al. have again demonstrated the power of optics to reveal fundamental information about superconductivity. This time, however, the revelations pertain to the underlying interactions in high-temperature superconductivity.
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