Anisotropic excitonic effects in the energy loss function of hexagonal boron nitride

TL;DR

This paper investigates the anisotropic excitonic effects in the energy loss function of hexagonal boron nitride, revealing significant directional dependence in energy, shape, and dispersion due to electron-hole interactions.

Contribution

It demonstrates the strong anisotropy in the valence energy-loss function of hBN and links it to electron-hole interactions and band-structure effects, which previous single-particle models could not capture.

Findings

Energy shift of 0.7 eV between directions

Splitting of the π-plasmon in the M direction

Anisotropic shape and dispersion in energy loss function

Abstract

We demonstrate that the valence energy-loss function of hexagonal boron nitride (hBN) displays a strong anisotropy in shape, excitation energy and dispersion for momentum transfer q parallel or perpendicular to the hBN layers. This is manifested by e.g. an energy shift of 0.7 eV that cannot be captured by single-particle approaches and is a demonstration of a strong anisotropy in the two-body electron-hole interaction. Furthermore, for in-plane directions of q we observe a splitting of the -plasmon in the M direction that is absent in the K direction and this can be traced back to band-structure effects.