Electronic aspects of the ferromagnetic transition in manganese perovskites

We have investigated the electronic structure associated with the ferromagnetic transition of La_{1 − x} Ca_x MnO₃ and La_{1 − x} Pb_x MnO₃ using high resolution photoemission, Mn 2p resonant photoemission, and O 1s x-ray absorption spectroscopies. The data clearly show that the band gap collapses 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 ∼ 1.5 eV, are responsible for the insulating behavior above T_C which has been enigmatic in the double-exchange model.