A reexamination of the effective fine structure constant of graphene, as measured in graphite

TL;DR

This study refines measurements of the effective fine structure constant in graphite, revealing stronger screening effects than previously thought, primarily due to excitonic effects and sigma-band screening, with implications for short-range interactions in graphene.

Contribution

The paper provides a more accurate measurement of the effective fine structure constant in graphite, highlighting the roles of excitonic effects and sigma-band screening, and compares results with RPA calculations.

Findings

Effective fine structure constant ranges from 0.25 to 0.35.

Sigma-band screening is highly effective at short distances.

Reduced alpha* is due to excitonic effects and sigma-band screening, not interlayer hopping.

Abstract

We present a refined and improved study of the influence of screening on the effective fine structure constant of graphene, $\alpha^*$, as measured in graphite using inelastic x-ray scattering. This follow-up to our previous study [J. P. Reed, et al., Science 330, 805 (2010)] was carried out with two times better energy resolution, five times better momentum resolution, and improved experimental setup with lower background. We compare our results to RPA calculations and evaluate the relative importance of interlayer hopping, excitonic corrections, and screening from high energy excitations involving the $\sigma$ bands. We find that the static, limiting value of $\alpha^*$ falls in the range 0.25 to 0.35, which is higher than our previous result of 0.14, but still below the value expected from RPA. We show the reduced value is not a consequence of interlayer hopping effects, which were ignored in our previous analysis, but of a combination of excitonic effects in the $\pi \rightarrow \pi^*$ particle-hole continuum, and background screening from the $\sigma$-bonded electrons. We find that $\sigma$-band screening is extremely strong at distances of the order of a few nm, and should be highly effective at screening out short-distance, Hubbard-like interactions in graphene, as well as other carbon allotropes.