Excitonic effects in energy loss spectra of freestanding graphene

TL;DR

This study uses high-resolution electron energy-loss spectroscopy and advanced theoretical models to analyze excitonic effects in the energy loss spectra of freestanding graphene, highlighting the importance of many-body interactions.

Contribution

It demonstrates the necessity of including both quasi-particle corrections and excitonic effects via GW and Bethe-Salpeter methods for accurate spectral description.

Findings

Excitonic effects significantly influence the excitation gap and $\pi$ plasmon.

Theoretical models with many-body effects match experimental spectra.

Quasi-particle and excitonic effects are essential for accurate EELS analysis.

Abstract

In this work we perform electron energy-loss spectroscopy (EELS) of freestanding graphene with high energy and momentum resolution to disentangle the quasielastic scattering from the excitation gap of Dirac electrons close to the optical limit. We show the importance of many-body effects on electronic excitations at finite transferred momentum by comparing measured EELS with ab initio calculations at increasing levels of theory. Quasi-particle corrections and excitonic effects are addressed within the GW approximation and Bethe-Salpeter equation, respectively. Both effects are essential in the description of the EEL spectra to obtain a quantitative agreement with experiments, with the position, dispersion, and shape of both the excitation gap and the $\pi$ plasmon being significantly affected by excitonic effects.