Lucia Reining: Exciton dispersion in low density systems: Explanations, predictions, and potential consequences

Exciton dispersion in low density systems: Explanations, predictions, and potential consequences

Abstract: Excitation spectra of valence electrons are often influenced by interaction effects, even for materials that are not considered to be strongly correlated. In particular, excitonic effects may dominate absorption spectra. In this talk, we start by reminding how the ab initio solution of the electron-hole problem allows one to understand excitonic effects in semiconductors and insulators that go beyond textbook exciton models. We then compare absorption spectroscopy with electron energy loss and inelastic x-ray scattering, and we show some surprising excitonic effects at large momentum transfer at the example of solid helium. Finally, we will concentrate on the low density regime of the homogeneous electron gas in 3 and 2 dimensions. We will discuss in which way excitonic effects may occur even in metals, in spite of perfect macroscopic screening. Potential experimental fingerprints of our predictions will be highlighted throughout the talk. The results that will be presented have mostly been obtained in collaboration with members of the Palaiseau Theoretical Spectroscopy Group and of the ETSF, and with colleagues from experimental groups.