Spectroscopic evidence for formation of small polarons in doped manganites

Temperature dependence of absorption spectra for thin films of doped manganites $R_{0.6}{\mathrm{Sr}}_{0.4}{\mathrm{MnO}}_3,$ 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 $\sim 1.5\mathrm{eV}$ 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 $(T>~T_{\mathrm{C}})$ changes into bare electrons (or large polarons) in the ferromagnetic state due to the enhanced one-electron bandwidth $W.\mathrm{}$ We further derived important physical quantities, i.e., $W,$ on-site exchange interaction $J,$ and binding energy $E_{\mathrm{p}}$ of SP, and discuss the material dependence of the stability of SP.