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Chemistry Seminar presented by Prof. Matthias Scheffler: "Electron-Vibrational Coupling in and beyond The Phonon Picture: Concepts and Applications to Thermal and Electrical Conductivity"
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Thursday, April 20, 2023 - 11:40am to 12:40pm
Prof. Matthias Scheffler, The NOMAD Laboratory at the FHI of the Max-Planck-Gesellschaft
"Electron-Vibrational Coupling in and beyond The Phonon Picture: Concepts and Applications to Thermal and Electrical Conductivity" Strong anharmonicity and its critical effect on materials properties are more frequent than typically presumed. Thus, for many materials and realistic temperatures the phonon picture and with that all phonon-based theories break down, so that an explicit treatment of the nuclear motion in terms of ab initio molecular dynamics becomes decisive for the description and understanding of many fundamental material properties, including thermal and electrical conductivities. In this talk we present a high-throughput search for thermal insulators employing a hierarchical workflow. It starts with a new anharmonicity measure that allows to rapidly cover thousands of materials so to single out potentially good candidates. The thermal conductivity of the 40 most promising compounds is then calculated with the ab initio Green-Kubo formalism, which accounts for anharmonicity to all orders. Moreover, we use the SISSO method to build an accurate, interpretable artificial-intelligence model for the lattice thermal conductivity, so to further accelerate material space exploration. By this means, we are able to identify several materials with ultra-low conductivity at room temperature. We analyze the actuating mechanisms that hinder heat transport in these materials, which in turn sheds light on the microscopic origin of strongly anharmonic effects. In particular, this demonstrates that energetically barely accessible defect configurations can induce a breakdown of the phonon picture. Eventually, we discuss how these anharmonic effects also influence the vibronic coupling to electronic degrees of freedom and discuss how this affects the electrical conductivity of strongly anharmonic systems.