Exciton energy-momentum map of hexagonal boron nitride

TL;DR

This paper presents a detailed exciton energy-momentum map of hexagonal boron nitride using Bethe-Salpeter equation calculations validated by inelastic x-ray scattering, revealing strong excitonic effects across momentum ranges.

Contribution

It provides the first comprehensive exciton energy-momentum map of h-BN validated by experiments, demonstrating the accuracy of BSE in complex materials.

Findings

Strong excitonic effects at large momentum transfer

Validation of BSE predictions with IXS data

Resolution of previous experimental discrepancies

Abstract

Understanding and controlling the way excitons propagate in solids is a key for tailoring materials with improved optoelectronic properties. A fundamental step in this direction is the determination of the exciton energy-momentum dispersion. Here, thanks to the solution of the parameter-free Bethe- Salpeter equation (BSE), we draw and explain the exciton energy-momentum map of hexagonal boron nitride (h-BN) in the first three Brillouin zones. We show that h-BN displays strong excitonic effects not only in the optical spectra at vanishing momentum $\mathbf{q}$, as previously reported, but also at large $\mathbf{q}$. We validate our theoretical predictions by assessing the calculated exciton map by means of an inelastic x-ray scattering (IXS) experiment. Moreover, we solve the discrepancies between previous experimental data and calculations, proving then that the BSE is highly accurate through the whole momentum range. Therefore, these results put forward the combination BSE and IXS as the tool of choice for addressing the exciton dynamics in complex materials.