SINGLE-PARTICLE AND OPTICAL GAPS IN CHARGE-TRANSFER INSULATORS

We analyze the collective excitations near a Mott-Hubbard and a metal charge transfer insulator transition, using the slave boson technique. We show that the Mott transition can be viewed as an excitonic softening, which takes place when the bound state between the lower and upper Hubbard bands reaches zero energy. The exciton energy is related to the jump of the chemical potential at zero doping. In a charge transfer insulator this mode describes a p-d charge fluctuation i.e. it is a charge transfer exciton. In the single band Hubbard model the excitonic resonance describes virtual transitions between the lower and the upper Hubbard band. Finally we contrast the behaviour of the collective modes near the Mott transition with and near the Charge Transfer Instability. In the former the exciton energy and the charge compressibility go to zero. In the latter the exciton energy remains finite and the charge susceptibility diverges, causing phase separation.