Seeing many-body effects in single- and few-layer graphene: Observation of two-dimensional saddle-point excitons

TL;DR

This study reveals strong many-body excitonic effects in graphene, observing a prominent saddle-point exciton resonance that is red-shifted from theoretical predictions, with minimal dependence on layer thickness.

Contribution

First experimental observation of saddle-point excitons in graphene, demonstrating many-body effects and their weak dependence on layer number.

Findings

Excitonic resonance at 4.62 eV observed in graphene.

Resonance is red-shifted by nearly 600 meV from GW calculations.

Weak dependence of excitonic effects on layer thickness.

Abstract

Significant excitonic effects were observed in graphene by measuring its optical conductivity in a broad spectral range including the two-dimensional {\pi}-band saddle-point singularities in the electronic structure. The strong electron-hole interactions manifest themselves in an asymmetric resonance peaked at 4.62 eV, which is red-shifted by nearly 600 meV from the value predicted by ab-initio GW calculations for the band-to-band transitions. The observed excitonic resonance is explained within a phenomenological model as a Fano interference of a strongly coupled excitonic state and a band continuum. Our experiment also showed a weak dependence of the excitonic resonance in few-layer graphene on layer thickness. This result reflects the effective cancellation of the increasingly screened repulsive electron-electron (e-e) and attractive electron-hole (e-h) interactions.