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, $\alpha^{\ast}_{g}(\mathbf{k},\omega)$, the value of which approaches $1/7$ 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.