The effective fine structure constant of freestanding graphene measured in graphite

TL;DR

This study measures the effective fine structure constant in freestanding graphene, revealing weaker interactions than expected due to excitonic effects, which influence charge screening and quasiparticle interactions.

Contribution

The paper provides the first experimental measurement of the scale-dependent effective fine structure constant in freestanding graphene, highlighting the role of excitonic effects in charge screening.

Findings

Effective fine structure constant approaches ~1/7 at low energies

Screening of quasiparticle interactions is enhanced by excitonic effects

Graphene exhibits weaker Coulomb interactions than previously thought

Abstract

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