Dirac cones reshaped by interaction effects in suspended graphene

In graphene, electron-electron interactions are expected to play a significant role, as the screening length diverges at the charge neutrality point and the conventional Landau theory that enables us to map a strongly interacting electronic liquid into a gas of non-interacting fermions is no longer applicable(1,2). This should result in considerable changes in graphene's linear spectrum, and even more dramatic scenarios, including the opening of an energy gap, have also been proposed(3-5). Experimental evidence for such spectral changes is scarce, such that the strongest is probably a 20% difference between the Fermi velocities v(F) found in graphene and carbon nanotubes(6). Here we report measurements of the cyclotron mass in suspended 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 near the neutrality point, and v(F) describing its slope increases by a factor of more than two and can reach approximate to 3 x 10(6) ms(-1) at n < 10(10) cm(-2). No gap is found at energies even as close to the Dirac point as similar to 0.1 meV. The observed spectral changes are well described by the renormalization group approach, which yields corrections logarithmic in n.