### Session Q30: Graphene: Electron-Electron Interactions

11:15 AM–2:15 PM, Wednesday, March 23, 2011
Room: C147/154

Chair: Bruno Uchoa, University of Illinois at Urbana-Champaign

Abstract ID: BAPS.2011.MAR.Q30.1

### Abstract: Q30.00001 : The effective fine-structure constant of freestanding graphene measured in graphite

11:15 AM–11:27 AM

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#### Authors:

Yu Gan
(University of Illinois at Urbana-Champaign)

James Reed
(University of Illinois at Urbana-Champaign)

Bruno Uchoa
(University of Illinois at Urbana-Champaign)

Young-Il Joe
(University of Illinois at Urbana-Champaign)

Diego Casa
(Argonne National Laboratory)

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.