Electronic structure of CuGeO3: Charge excitations between zero and one dimension - art. no. 075112

We present a joint experimental and theoretical investigation of the electr onic structure of CuGeO3. The momentum dependent loss function was measured using electron energy-loss spectroscopy in transmission. For momentum tran sfers parallel to the crystallographic c direction (along the CuO2 chains) the loss function a.-roes well with calculations based upon a Cu5O12 cluste r model. The detailed analysis of the origin of the features below 7 eV in the calculated spectra reveals two distinct energy ranges in which transiti ons occur either into localized or delocalized states. A shift of spectral weight towards the low energy region can be observed when the coupling betw een adjacent CuO4 plaquettes is increased in our model. Above 2 eV the opti cal conductivity derived from the experimental loss function agrees well wi th results from optical measurements as well as with those from calculation s 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 t hus cannot be assigned to charge transfer transitions involving Cu and O st ates. The loss functions with momentum transfers parallel to the crystallog raphic b direction show spectral weight at 6.5 eV that shifts to higher ene rgies with increasing momentum transfer. This can probably be ascribed to t ransitions involving unoccupied Ge 4s and 4p states.