Exciton-Phonon Coupling from First Principles

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Optical properties of materials are most relevant for a large variety of technological applications, ranging from photovoltaics over various spectroscopy techniques to LEDs and displays. In optical spectra, microscopic quantum many-body effects like excitons, i.e. coupled electron-hole-pair excitations, are measurable at a macroscopic scale and crucially determine the materials properties. Hence, a deep understanding of excitons constitutes an indispensable driving force for innovation in optics and optoelectronics. The state-of-the-art parameter-free theoretical description of excitons is based on the ab-initio methods of many-body perturbation theory. The present theoretical standard approach for exciton spectra, which relies on the Bethe-Salpeter equation, takes only the static electronic screening of the electron-hole-pair interaction into account. The coupling of excitons to phonons and, hence, polaronic screening contributions are completely omitted. However, the exciton-phonon coupling is crucial for the qualitative and quantitative understanding of exciton spectra in materials with strong polaronic effects, such as many technologically highly relevant oxides. EXPHON will close this gap of knowledge. It focuses on the ab-initio description of excitons in systems with strong polaronic lattice screening. The key objectives are (i) the development of a theoretical method that takes polaronic screening of the electron-hole interaction into account, (ii) the implementation of this theoretical scheme in an ab-initio software package, and (iii) the calculation of reliable exciton spectra for materials with strong polaronic effects. This will be a major step forward towards a full description of the interplay between electronic and lattice degrees of freedom in semiconductors.