  Fano resonances and their strong doping dependence are observed in Raman
scattering of single-layer graphene (SLG). As the Fermi level is varied by a
back-gate bias, the Raman G band of SLG exhibits an asymmetric line shape near
the charge neutrality point as a manifestation of a Fano resonance, whereas the
line shape is symmetric when the graphene sample is electron or hole doped.
However, the G band of bilayer graphene (BLG) does not exhibit any Fano
resonance regardless of doping. The observed Fano resonance can be interpreted
as interferences between the phonon and excitonic many-body spectra in SLG. The
absence of a Fano resonance in the Raman G band of BLG can be explained in the
same framework since excitonic interactions are not expected in BLG.
