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 [r(A)] of perovskite A site. We have observed a specific absorption band at similar to 1.5 eV 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 greater than or equal to T-C) changes into bare electrons (or large polarons) in the ferromagnetic state due to the enhanced one-electron bandwidth TV. We further derived important physical quantities, i.e., W, on-site exchange interaction J, and binding energy E-p Of SP, and discuss the material dependence of the stability of SP.