Manifestation of nonlocal electron-electron interaction in graphene

Graphene is an ideal platform to study many-body effects due to its semimetallic character and the possibility to dope it over a wide range. Here we study the width of graphene's occupied $\ensuremath{\pi}$ band as a function of doping using angle-resolved photoemission. Upon increasing electron doping, we observe the expected shift of the band to higher binding energies. However, this shift is not rigid and the bottom of the band moves less than the Dirac point. We show that the observed shift cannot be accounted for by single-particle effects and local self-energies alone, but that nonlocal many-body effects, in particular exchange interactions, must be taken into account.