Lawrence Berkeley National Laboratory

A number of interesting properties of graphene and graphite are postulated to derive from the peculiar bandstructure of graphene. This bandstructure consists of conical electron and hole pockets that meet at a single point in momentum (k) space--the Dirac crossing, at energy $E_{D} = \hbar \omega_{D}$. Direct investigations of the accuracy of this bandstructure, the validity of the quasiparticle picture, and the influence of many-body interactions on the electronic structure have not been addressed for pure graphene by experiment to date. Using angle resolved photoelectron spectroscopy (ARPES), we find that the expected conical bands are distorted by strong electron-electron, electron-phonon, and electron-plasmon coupling effects. The band velocity at $E_{F}$ and the Dirac crossing energy $E_{D}$ are both renormalized by these many-body interactions, in analogy with mass renormalization by electron-boson coupling in ordinary metals. These results are of importance not only for graphene but also graphite and carbon nanotubes which have similar bandstructures.