Dirac cones reshaped by interaction effects in suspended graphene
  We report measurements of the cyclotron mass in graphene for carrier concentrations n varying over three orders of magnitude. In contrast to the single-particle picture, the real spectrum of graphene is profoundly nonlinear so that the Fermi velocity describing the spectral slope reaches ~3x10^6 m/s at n <10^10 cm^-2, three times the value commonly used for graphene. The observed changes are attributed to electron-electron interaction that renormalizes the Dirac spectrum because of weak screening. Our experiments also put an upper limit of ~0.1 meV on the possible gap in graphene. 
D. C. Elias, R. V. Gorbachev, A. S. Mayorov, S. V. Morozov, A. A.   Zhukov, P. Blake, L. A. Ponomarenko, I. V. Grigorieva, K...
cond-mat.mes-hall cond-mat.mtrl-sci cond-mat.str-el