Anomalous Angular Dependence of the Dynamic Structure Factor near Bragg Reflections: Graphite

TL;DR

This study reveals that the dynamic structure factor of graphite exhibits significant angular dependence near Bragg reflections, influenced by local field effects and stacking, with implications for understanding electron energy-loss spectra.

Contribution

The paper combines first-principles calculations and inelastic x-ray scattering to uncover the angular dependence of the dynamic structure factor near Bragg reflections in graphite, highlighting the role of local field effects.

Findings

Spectra change drastically with small variations in momentum transfer near Bragg reflections.

The effect is governed by crystal local field effects and stacking of graphite.

Strong coupling between excitations at different momentum transfers is observed.

Abstract

The electron energy-loss function of graphite is studied for momentum transfers q beyond the first Brillouin zone. We find that near Bragg reflections the spectra can change drastically for very small variations in q. The effect is investigated by means of first principle calculations in the random phase approximation and confirmed by inelastic x-ray scattering measurements of the dynamic structure factor S(q,\omega). We demonstrate that this effect is governed by crystal local field effects and the stacking of graphite. It is traced back to a strong coupling between excitations at small and large momentum transfers.