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Thursday, February 8, 2018 - 11:15 to 12:30 Auditorium
Quantum Material, as exemplified by unconventional superconductors and topological insulators, is a fascinating and rapidly developing field of modern physics. High-temperature superconductivity in cupper and iron based materials, with critical temperature well above what was anticipated by the BCS, remains a major unsolved physics problem today. The challenge of this problem is symbolized by a complex phase diagram consists of intertwined states with extreme 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 as these states are different manifestations of the same underlying physical system, making an integrated understanding a necessity. Angle-resolved photoemission spectroscopy (ARPES), derived from Einstein’s formulation of photoelectric effect, has emerged as a leading experimental tool to push the frontier of this important field of modern physics. Over the last three decades, the improved resolution and carefully matched experiments have been the keys to turn this technique into a sophisticated many-body physics tool. As a result, ARPES played a critical role in setting the intellectual agenda by testing new ideas and discovering surprises. ARPES has impacted both the field of unconventional superconductors and topological phases of matter. In this talk, we discuss ARPES evidence for a general theme of high temperature superconductivity - cooperative enhancement and positive feedback loop of different interactions exemplified by electron-electron and electron-phonon interactions. The accumulated evidence comes from an expanded version of angle-resolved photoemission spectroscopy and its match to in-situ material synthesis. In such experiments, the precision measurements of electron’s energy, momentum and time dynamics provide evidence for cooperative interactions as a pathway to increase the superconducting transition temperature. An outlook for ARPES development and application for other quantum materials will also be discussed.