Inelastic x-ray scattering studies of low-energy charge excitations in graphite

We have studied charge excitations in graphite using inelastic x-ray scattering. The spectra were measured as a function of the momentum transfer, q, along the [100] (k(x)), [210] (k(y)), and [001] (k(z)) axes on a high-quality single-crystal sample. Anisotropic and periodic features representing the semimetallic electronic structure have been observed. These features are interpreted based on band structure obtained from the local density approximation. On the q parallel to k(x) and q parallel to k(y) spectra, periodic signals exhibiting the gap opening and closing of the pi bands have been observed. On the other hand, on q parallel to k(z) spectra, a forbidden excitation between the pi bands has been detected in the high q region.