  We present magneto-Raman scattering studies of electronic inter Landau level
excitations in quasi-neutral graphene samples with different strengths of
Coulomb interaction. The band velocity associated with these excitations is
found to depend on the dielectric environment, on the index of Landau level
involved, and to vary as a function of the magnetic field. This contradicts the
single-particle picture of non-interacting massless Dirac electrons, but is
accounted for by theory when the effect of electron-electron interaction is
taken into account. Raman active, zero-momentum inter Landau level excitations
in graphene are sensitive to electron-electron interactions due to the
non-applicability of the Kohn theorem in this system, with a clearly
non-parabolic dispersion relation.
