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, α*(k,ω), whose value approaches α* ~ 1/7 at low energy and large distances. This value is substantially smaller than the nominal α= 2.2, suggesting that, on the whole, graphene is more weakly interacting than previously believed.

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