Reexamination of the effective fine structure constant of graphene as measured in graphite

We present a refined and improved study of the influence of screening on the effective fine structure constant of graphene, α∗, as measured in graphite using inelastic x-ray scattering. This followup to our previous study [J. P. Reed, Science 330, 805 (2010)SCIEAS0036-807510.1126/science.1190920] was carried out with two times better energy resolution, five times better momentum resolution, and an improved experimental setup with lower background. We compare our results to random-phase approximation (RPA) calculations and evaluate the relative importance of interlayer hopping, excitonic corrections, and screening from high energy excitations involving the σ bands. We find that the static, limiting value of α∗ falls in the range 0.25-0.35, 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 π→π∗ particle-hole continuum, and background screening from the σ-bonded electrons. We find that σ-band screening is extremely strong at distances of less than a few nanometers, and should be highly effective at screening out short-distance, Hubbard-like interactions in graphene as well as other carbon allotropes.