A. G. Marinopoulos

Anomalous angular dependence of the dynamic structure factor near Bragg reflections: graphite.

The electron energy-loss function of graphite is studied for momentum transfers q beyond the first Brillouin zone. We find that near Bragg reflections the spectra can change drastically for very small variations in q. The effect is investigated by means of first principle calculations in the random phase approximation and confirmed by inelastic x-ray scattering measurements of the dynamic structure factor S(q, omega). We demonstrate that this effect is governed by crystal local field effects and the stacking of graphite. It is traced back to a strong coupling between excitations at small and large momentum transfers.

Optical absorption in small BN and C nanotubes

We present a theoretical study of the optical absorption spectrum of small boron‐nitride and carbon nanotubes using time‐dependent density‐functional theory and the random phase approximation. Both for C and BN tubes, the absorption of light polarized perpendicular to the tube‐axis is strongly suppressed due to local field effects. Since BN‐tubes are wide band‐gap insulators, they only absorb in the ultra‐violet energy regime, independently of chirality and diameter. In comparison with the spectra of the single C and BN‐sheets, the tubes display additional fine‐structure which stems from the (quasi‐) one‐dimensionality of the tubes and sensitively depends on the chirality and tube diameter. This fine structure can provide additional information for the assignment of tube indices in high resolution optical absorption spectroscopy.