Optical phase diagram of perovskite colossal magnetoresistance manganites near half doping

We present a systematic optical study for a bandwidth-controlled series of nearly half-doped colossal magnetoresistive manganites RE(0.55)AE(0.45)MnO(3) (RE and AE being rare earth and alkaline earth ions, respectively) under the presence of quenched disorder over a broad temperature region T=10-800 K. The ground state of the compounds ranges from the charge and orbital ordered insulator through the spin glass to the ferromagnetic metal. The enhanced phase fluctuations, namely, the short-range charge and orbital correlations, dominate the paramagnetic region of the phase diagram above all the ground-state phases. This paramagnetic region is characterized by a full-gap to pseudogap crossover toward elevated temperatures where a broad low-energy electronic structure appears in the conductivity spectra over a large variation of the bandwidth. This pseudogap state with local correlations is robust against thermal fluctuations at least up to T=800 K. For a small bandwidth, the onset of the long-range charge order is accompanied by an instantaneous increase of the gap. The emergence of the ferromagnetic state is manifested in the optical spectra as a first-order insulator to a metal transition for compounds with a moderate bandwidth, while it becomes a second-order transition on the larger bandwidth side. An unusually large scattering rate of the metallic carriers is observed in the ferromagnetic state, which is attributed to orbital correlation effects.