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 pairs, which belong to the irreducible representation A$_2$ of the point group C$_{6v}$ of the graphene lattice, and are characterised by crossed polarisation of incoming and outgoing photons. At high magnetic fields, this is manifested by the excitation of electron-hole (e-h) inter-Landau-level transitions with selection rule $n^-\to n^+$. Weaker Raman-active inter-Landau-level modes also exist. One of those has a selection rule similar to the infrared absorption process, $n^-\to(n\pm1)^+$, 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 strong magnetic field. The fine structure of 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.