SPECTROSCOPIC EVIDENCE FOR FORMATION OF SMALL POLARONS IN DOPED MANGANITES

Temperature dependence of absorption spectra for thin films of doped m anganites R0.6Sr0.4MnO3, where R is a rare-earth atom, has been invest igated by systematically changing the averaged ionic radius [r(A)] of perovskite A site. We have observed a specific absorption band at simi lar to 1.5 eV due to optical excitations from small polarons (SP). Spe ctral weight of the SP band increases with decreasing temperature and eventually disappears at the insulator-metal transition, indicating th at SP in the paramagnetic state (T greater than or equal to T-C) chang es into bare electrons (or large polarons) in the ferromagnetic state due to the enhanced one-electron bandwidth TV. We further derived impo rtant physical quantities, i.e., W, on-site exchange interaction J, an d binding energy E-p Of SP, and discuss the material dependence of the stability of SP.