Photoemission Spectroscopy of the High-Temperature Superconductivity Gap

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Abstract
Superconductivity is related to the presence of a narrow forbidden gap in the spectrum of the possible energies for the electrons in the material. These "superconductivity gaps" have traditionally been studied with tunneling and infrared absorption experiments. A third, powerful technique has been made possible by the discovery of hightransition temperature materials: the direct observation of the gap in photoemission spectra. The data analysis requires a careful reconsideration of the standard Einstein-Fermi model of the photoelectric effect. The conclusions are surprisingly simple and offer an alternate way to measure superconductivity gaps. This approach can also be used to study the directional properties of the gap, phenomena related to the coherence length, and possible departures from Fermi-liquid behavior.