Open Access

István Kézsmárki, Y. Tomioka, S. Miyasaka, L. Demkó, Y. Okimoto, Y. Tokura

• We present a systematic optical study for a bandwidth-controlled series of nearly half-doped colossal magnetoresistive manganites RE0.55AE0.45MnO3 (RE and AE being rare earth and alkaline earth ions, respectively) under the presence of quenched disorder over a broad temperature region T=10–800K. 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=800K. For a small bandwidth, the onset of theWe present a systematic optical study for a bandwidth-controlled series of nearly half-doped colossal magnetoresistive manganites RE0.55AE0.45MnO3 (RE and AE being rare earth and alkaline earth ions, respectively) under the presence of quenched disorder over a broad temperature region T=10–800K. 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=800K. 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.…