Excitonic Effects in Energy-Loss Spectra of Freestanding Graphene
Alberto Guandalini, Ryosuke Senga, Yung‐Chang Lin, Kazu Suenaga, Andrea Ferretti, Daniele Varsano, Andrea Recchia, Paolo Barone, Francesco Mauri, Thomas Pichler, Christian Kramberger
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 to ab initio calculations at increasing levels of theory. Quasi-particle corrections and excitonic effects are addressed within the GW approximation and the 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 π plasmon being significantly affected by excitonic effects.