Wave Vector Dependence of the Electron Energy Loss Functions of Graphite

Electron energy loss measurements were performed on graphite single crystals with 55 keV electrons. From spectra taken with transferred momentum hq parallel to the c‐axis and recorded for different absolute values of |q| the wave vector dependent energy loss function Im [−l/ϵ‖( E , q )] was determined for 0< E ⪅ 8 eV and 0.25 Å −1 <|q|< < 0.75 Å −1 . Apart from the dispersion of the energy loss maximum in the 5 to 6 eV region the main feature of Im [−1/ϵ‖( E , q )] is the appearence of a band gap E g , increasing from E g ≪ 1 eV for |q| = 0 up to E g ≈︁ 1 eV for |q| as 0.7 Å −1 . Additional measurements made with transferred momentum ħ q perpendicular to the c‐axis confirmed the dispersion of the π‐plasmon, exhibiting an anisotropy for large wave vectors and different orientation of the latter within the layer planes of graphite.