Near Infrared Reflectivity of Graphite under Hydrostatic Pressure. I. Experiment

We report here the dependence on hydrostatic pressure up to 2.79 kbars of the two structures associated with the A₁ and A₂ transitions in the reflectivity spectra of graphite measured at room temperature in the photon energy range from 0.655 to 1.125 eV. By considering explicitly the effect of the trigonal warping upon the probabilities of these transitions, their spectral shapes are shown to be a step and a peak, respectively, resulting in new locations of critical points on the spectral features A₁ and A₂ which are different from those previously given by Bellodi et al The observed shifts of these structures due to pressure provide the first direct experimental determination of the fractional change per unit pressure increment of the Slonczewski-Weiss-McClure band parameters γ₁ and γ₅, yielding values of ∂ In γ₁/∂p=0.028±0.003 (kbar)^-1 and ∂ In γ₅/∂p=0.055± 0.016 (kbar)^-1.