Coherent-incoherent crossover of charge dynamics in the ferromagnetic ground state of manganites: $R_{0.6}{\mathrm{Sr}}_{0.4}{\mathrm{MnO}}_3$ $(R=\mathrm{La}$-Sm)

Low-energy optical spectra and their temperature dependence have been investigated for single crystals of perovskite-type manganese oxides, $R_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3,$ at a fixed doping level $\mathrm{}(x=0.4\mathrm{})\mathrm{}$ but with varying perovskite R species or one-electron bandwidth $\mathrm{}\left(W\right).$ Variation of the optical conductivity spectra with a change of R, such as a diminishment of the Drude peak and the emergence of a nearly $\omega$-independent component, indicates a coherent-incoherent crossover of charge dynamics in the ferromagnetic metallic phase. This cannot be accounted for in terms of simple change in carrier mass or scattering rate, but indicates the importance of competitive instabilities triggered by decreasing W against the double-exchange interaction near the charge/orbital-ordering instability.