The resonance between the G-band phonon excitation and Landau level optical transitions in graphene has been systematically studied by micromagneto Raman mapping. In purely decoupled graphene regions on a graphite substrate, eight traces of anticrossing spectral features with G-mode peaks are observed as a function of magnetic fields up to 9 T, and these traces correspond to either symmetric or asymmetric Landau level transitions. Three distinct split peaks of the G mode, named $G_{-}$, $G_i$, and $G_{+}$, are observed at the strong magnetophonon resonance condition corresponding to a magnetic field of ∼4.65 T. These three special modes are attributed to (i) the coupling between the G phonon and the magneto-optical transitions, which is responsible for $G_{+}$ and $G_{-}$ and can be well described by the two coupled mode model and (ii) the magnetic field-dependent oscillation of the $G_i$ band, which is currently explained by the G band of graphite modified by the interaction with $G_{+}$ and $G_{-}$. The pronounced interaction between Dirac fermions and phonons demonstrates a dramatically small Landau level width (∼1.3 meV), which is a signature of the ultrahigh quality graphene obtained on the surface of graphite.

• Received 26 April 2013

DOI:https://doi.org/10.1103/PhysRevB.88.165407