Spectroscopic evidence for formation of small polarons in doped manganites

Temperature dependence of absorption spectra for thin films of doped manganites R0.6Sr0.4MnO₃, where R is a rare-earth atom, has been investigated by systematically changing the averaged ionic radius 〈rA〉 of perovskite A site. We have observed a specific absorption band at ~1.5eV due to optical excitations from small polarons (SP). Spectral weight of the SP band increases with decreasing temperature and eventually disappears at the insulator-metal transition, indicating that SP in the paramagnetic state (T>~TC) changes into bare electrons (or large polarons) in the ferromagnetic state due to the enhanced one-electron bandwidth $W.$ We further derived important physical quantities, i.e., $W,$ on-site exchange interaction $J,$ and binding energy Eₚ of SP, and discuss the material dependence of the stability of SP.