Dirac formalism for non-interacting massless fermions explains many assets of graphene, though it remains only an approximate model to accurately account for all its low energy properties. As compared to a genuine quantum electrodynamics system, the effective speed of light in graphene is reduced by a factor of 300, what among other things, implies that its electronic dispersion relations and low energy excitations can be significantly reshaped by electron-electron interactions. This is now confirmed with magneto-Raman scattering studies reported by the team of researchers from the Laboratoire des Champs Magnétiques Intenses-Grenoble, the Institut de Physicio-Chimie des Matériaux de Strasbourg, the Laboratoire de Physique et Modélisation de la Matière Condensée in Grenoble, the University of Manchester, Columbia University in New York and the University of Tsukuba in Japan. Working on three graphene systems with different strength of interaction - strong in suspended graphene, moderate in graphene encapsulated in boron nitride and nearly absent in graphene on graphite - and using the proper theoretical tools, the specific effects of electron-electron interactions in graphene could be analyzed and described accurately for the first time. These results are presented in Fig. 1, which shows the evolution of the carrier velocity (the slope of the electronic dispersion at zero magnetic field) as a function of the magnetic field in logarithmic scale. In the non-interacting picture, all these traces should appear as a single horizontal line. The observed effects of electron-electron interaction consist in the renormalization of the electronic dispersion relations as well as of the additional modification of electron-hole (inter Landau level) excitation energies due to the breaking of the Kohn theorem for the non-parabolic bands of graphene. This simple layer of carbon atoms appears as a new playground for many body physics, accessible with the contactless techniques of optical spectroscopy.
More on this topic : C. Faugeras et al., Physical Review Letters 114, 126804, (2015)
Fig. 1. Evolution of the “effective” carrier velocity as a function of the magnetic field in logarithmic scale, for three different graphene systems, suspended graphene (G-S), graphene encapsulated in boron nitride (G-BN) and graphene on graphite (G-Gr).