Abstract
We have investigated the electronic structure associated with the ferromagnetic transition of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ and ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Pb}}_{x}{\mathrm{MnO}}_{3}$ 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 $({T}_{C})$ 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 $\ensuremath{\sim}1.5$ eV, are responsible for the insulating behavior above ${T}_{C}$ which has been enigmatic in the double-exchange model.
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