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. In fact, for many materials and realistic temperatures, the observed structure and symmetry is a statistical average of energetically degenerate, lower-symmetry geometries. Here phonon-based perturbation theories break down, and an explicit treatment of the nuclear dynamics in terms of ab initio molecular dynamics becomes necessary to account for such strong anharmonic effects.

Accurately treating these effects is decisive for 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 [1] that allows to rapidly cover thousands of materials so to single out potential thermal insulators. The thermal conductivity of the most promising 40 candidates is then calculated with the ab initio Green-Kubo [2] formalism, which accounts for anharmonicity to all orders. Moreover, we use the SISSO method [3] to build an accurate, interpretable artificial intelligence model for the lattice thermal conductivity [4], 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. Eventually, we discuss how anharmonicity also influences the vibronic coupling to electronic degrees of freedom [5] and discuss how this affects the electrical conductivity of strongly anharmonic systems.