Electronic Aspects of the Ferromagnetic Transition in Manganese Perovskites

We have investigated the electronic structure associated with the ferromagnetic transition of ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ and ${\mathrm{La}}_{1-x}{\mathrm{Pb}}_x{\mathrm{MnO}}_3$ 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 $\left(T_C\right)$ 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 $\sim 1.5$ eV, are responsible for the insulating behavior above $T_C$ which has been enigmatic in the double-exchange model.