The Effective Fine-Structure Constant of Freestanding Graphene Measured in Graphite

Electrons in graphene behave like Dirac fermions, permitting phenomena from high-energy physics to be studied in a solid-state setting. A key question is whether or not these fermions are critically influenced by Coulomb correlations. We performed inelastic x-ray scattering experiments on crystals of graphite and applied reconstruction algorithms to image the dynamical screening of charge in a freestanding graphene sheet. We found that the polarizability of the Dirac fermions is amplified by excitonic effects, improving screening of interactions between quasiparticles. The strength of interactions is characterized by a scale-dependent, effective fine-structure constant, alpha*(g) (k, omega), the value of which approaches 0.14 +/- 0.092 similar to 1/7 at low energy and large distances. This value is substantially smaller than the nominal alpha(g) = 2.2, suggesting that, on the whole, graphene is more weakly interacting than previously believed.