CHARGED EXCITONS IN DOPED EXTENDED HUBBARD-MODEL SYSTEMS

We show that the charge transfer excitons in a Hubbard model system in cluding nearest-neighbor Coulomb interactions effectively attain some charge in doped systems and become visible in photoelectron and invers e photoelectron spectroscopies. This shows that the description of a d oped system by an extended Hubbard model differs substantially from th at of a simple Hubbard model. Longer-range Coulomb interactions cause satellites in the one-electron removal and addition spectra and the ap pearance of spectral weight in the gap of doped systems at energies co rresponding to the excitons of the undoped systems. The spectral weigh t of the satellites is proportional to the doping times the coordinati on number and therefore is strongly dependent on the dimension.