High-temperature superconductivity in copper and iron based materials, with critical temperature well above what was anticipated by the Bardeen-Copper-Schrieffer theory, remains a major unsolved physics problem today. The challange of this problem is symbolized by a complex phase diagram that consists of intertwined states with unusual properties in addition to unconventional superconductivity. None of them can be described by conventional theory, thus compounding the difficulty to understand high-temperature superconductivity itself. These states are different manifestations of the same underlying physicsl system that consists of multiple degrees of freedon, making an integrated understanding a necessity.
We discuss experimental evidence for a general theme in copper and iron based superconductors - cooperative enhancement and positive feedback loop of different interactions exemplified by electron-electron and electron-phonon interactions. The accumulated evidence comes from an expanded verson of angle-resolved photoemission spectroscopy and its match to in-situ material synthesis. In such experiments, the precision measurements of electrons' energy, momentum and time dynamics provide evidence for cooperative interplay of different degrees of freedon as a route to increase the effective interactions, and the resulting supersonducting transition temperature.
Speaker: Zhi-Xun Shen, Stanford
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