We present magneto-Raman scattering studies of electronic inter-Landau level excitations in quasineutral 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 noninteracting 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 nonapplicability of the Kohn theorem in this system, with a clearly nonparabolic dispersion relation.
Figure 1 : Magnetic field dependence (B0= 1 T) of the velocities associated with L−1 ;1 and L−2 ;2 inter-Landau level excitations shown with, correspondingly, open circles (solid lines) and open stars (dashed lines), as derived from the experiment (data modeling), for G-S, G-hBN, and G-Gr specimens. Straight lines are theoretical calculations, with ε = 3.9, 7.0, and 12 for G-S, G-hBN, and G-Gr species, respectively, and the corresponding values for εδv = 1.3, 3.7, and 12.
C. Faugeras et al. Phys. Rev. Lett. 114, 126804, (2015)