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 A2 of the point group C6v 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 E2 (rather than E1) 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. (paper)