Raman spectroscopy of graphite in high magnetic fields

Recently, much attention has been paid to electron-phonon coupling in graphene. In particular, significant re-normalization and broadening of long-wavelength optical phonons are predicted to occur through resonant interaction with Landau-quantized Dirac fermions. We report here on a high-field magneto-Raman spectroscopy study of highly-oriented pyrolytic graphite (HOPG) and natural graphite at temperatures down to 5 K and in magnetic fields up to 45 T. The E$_{2g}$ graphite phonon line exhibits anticrossing-like behavior at approximately 30 T, which we attribute to the magneto-phonon resonance (MPR) of graphite's massless holes at the H-point. Additionally, we observed features related to inter-Landau-level transitions at the K-point of graphite. We also observed weak graphene-like signatures of MPR, indicating the existence of graphene flakes on the graphite surface.