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

摘要

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