Anomalous angular anisotropy of excitonic spectra

The goal of the proposal is to validate the theoretical prediction of an anomalous angular anisotropy of excitonic peaks around Bragg reflections in the dynamical structure factor S(q,ω) measured by inelastic X-ray scattering (IXS) in materials with strongly bound excitons. The prediction will be verified in hBN, a prototypical layered compound characterised by strong excitonic effects, where previous combinations of IXS experiments and theoretical simulations have already proven their accuracy. The predicted anisotropy is due to the electron-hole exchange interaction and has a general validity in crystalline materials. The beamtime will demonstrate that its measurement by IXS is a direct way to quantify the impact of the electron-hole exchange interaction on electronic excitations.