Resonant Excitation of Graphene K-Phonon and Intra-Landau-Level Excitons   in Magneto-Optical Spectroscopy
  Precise infrared magnetotransmission experiments have been performed in magnetic fields up to 32 T on a series of multilayer epitaxial graphene samples. We observe changes in the spectral features and broadening of the main cyclotron transition when the incoming photon energy is in resonance with the lowest Landau level separation and the energy of a K point optical phonon. We have developed a theory that explains and quantitatively reproduces the frequency and magnetic field dependence of the phenomenon as the absorption of a photon together with the simultaneous creation of an intervalley, intra-Landau-level exciton, and a K phonon. 
M. Orlita, Liang Z. Tan, M. Potemski, M. Sprinkle, C. Berger, W. A. de   Heer, Steven G. Louie, and G. Martinez
cond-mat.mes-hall cond-mat.mtrl-sci cond-mat.str-el