Role of electronic excitations in magneto-Raman spectra of graphene

We investigate the signature of the low-energy electronic excitations in the Raman spectrum of monolayer and bilayer graphenes. The dominant contribution to the Raman spectra is due to the interband electron-hole (e-h) pairs, which belong to the irreducible representation A(2) of the point group C-6v of the graphene lattice, and are characterized by crossed polarization of incoming and outgoing photons. At high magnetic fields, this is manifested by the excitation of e-h inter-Landau-level (LL) transitions with selection rule n(-) -> n(+). Weaker Raman-active inter-LL modes also exist. One of those has a selection rule similar to the infrared absorption process, n(-) -> (n +/- 1)(+), but the created e-h excitation belongs to the irreducible representation E-2 (rather than E-1) and couples to the optical phonon mode, thus undergoing an anticrossing with the optical phonon G-line in Raman in a strong magnetic field. The fine structure acquired by the G-line due to such anticrossing depends on the carrier density, inhomogeneity of doping and presence of inhomogeneous strain in the sample.