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245 East Avenue
Shuolong Yang, Department of Physics, Stanford University, Stanford, CA, will present seminar. Professor Kyle Shen, host.
Seminar Title: Understanding electron-phonon interactions for quantum material engineering
Abstract: Imaging quantum many-body interactions is essential for understanding the complex phenomena inherent to quantum materials, and provides guidelines for engineering future material systems. In this seminar I will demonstrate unique perspectives from equilibrium and non-equilibrium photoemission spectroscopies using two model systems. First, we investigate superconducting calcium-intercalated graphite, and show that the distinct symmetries of the graphene-derived and intercalant-derived bands enable the coupling to the out-of-plane carbon mode. This mode enhances the total coupling strength by a factor of two, thus facilitating superconductivity in this compound as well as in monolayer graphene. Secondly, I will discuss FeSe thin films on SrTiO3 substrates. The superconducting Tc is enhanced by one order of magnitude in monolayer FeSe films (Tc > 70 K) as compared to bulk FeSe (Tc = 8 K). Using time-resolved photoemission spectroscopy, we image coherent oscillations of the electronic band structure. By combining this information with the lattice displacement measured by time-resolved X-ray diffraction, we are able to use a lock-in approach to reveal the inherent phonon frequency to precisely measure electron-phonon interactions. With the insights from these studies, I propose to engineer and visualize electron-phonon interactions on an atomic level using molecular beam epitaxy and angle-resolved photoemission spectroscopy. The subtle control of material properties will enable novel material concepts such as FeSe-oxide hetero-interface, which holds the promise of record-breaking Tc’s.
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