Electronic structure of CuGeO 3: Charge excitations between zero and one dimension: Charge excitations between zero and one dimension

We present a joint experimental and theoretical investigation of the electronic structure of CuGeO3. The momentum dependent loss function was measured using electron energy-loss spectroscopy in transmission. For momentum transfers parallel to the crystallographic c direction (along the CuO2 chains) the loss function agrees well with calculations based upon a Cu5O12 cluster model. The detailed analysis of the origin of the features below 7 eV in the calculated spectra reveals two distinct energy ranges in which transitions occur either into localized or delocalized states. A shift of spectral weight towards the low energy region can be observed when the coupling between adjacent CuO4 plaquettes is increased in our model. Above 2 eV the optical conductivity derived from the experimental loss function agrees well with results from optical measurements as well as with those from calculations based on our cluster model. The small spectral weight observed at 1.8 eV in the experiment has no counterpart in the theoretical loss function and thus cannot be assigned to charge transfer transitions involving Cu and O states. The loss functions with momentum transfers parallel to the crystallographic b direction show spectral weight at 6.5 eV that shifts to higher energies with increasing momentum transfer. This can probably be ascribed to transitions involving unoccupied Ge 4s and 4p states.