ELECTRONIC ASPECTS OF THE FERROMAGNETIC TRANSITION IN MANGANESE PEROVSKITES

We have investigated the electronic structure associated with the ferr omagnetic transition of La1-xCaxMnO3 and La1-xPbxMnO3 using high resol ution photoemission, Mn 2p resonant photoemission, and O 1s x-ray abso rption spectroscopies. The data clearly show that the band gap collaps es below the Curie temperature (T-C) and the density of states at the Fermi level increases with cooling, providing a conclusive microscopic evidence for an insulator-metal transition. Our results suggest that strong small polaron effects, which contribute to a charge fluctuation energy of similar to 1.5 eV, are responsible for the insulating behav ior above T-C which has been enigmatic in the double-exchange model.