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

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Science  05 Nov 2010:
Vol. 330, Issue 6005, pp. 805-808
DOI: 10.1126/science.1190920

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Weakly Interacting Graphene

Many unusual properties of graphene are a consequence of the Dirac dispersion of its electrons—a linear relationship between an electron's momentum and energy. Naïvely, this dispersion leads to the conclusion that electrons in graphene are strongly affected by mutual electrostatic interactions; however, there is little experimental evidence for strong interaction. Reed et al. (p. 805) resolved this discrepancy by using inelastic x-ray scattering spectra of graphite (which consists of loosely bound layers of graphene) to estimate how much the electric field was damped by the presence of mobile charge carriers. In fact, damping was strong at distances in excess of 1 nanometer, suggesting that graphene is more weakly interacting than was assumed.


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, Embedded Image, the value of which approaches Embedded Image at low energy and large distances. This value is substantially smaller than the nominal Embedded Image, suggesting that, on the whole, graphene is more weakly interacting than previously believed.

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