Optical conductivity of colossal-magnetoresistance compounds: Role of orbital degeneracy in the ferromagnetic phase

Recent optical conductivity sigma(omega) experiments have revealed anomalous spectral distribution in the ferromagnetic phase of the perovskite system La1-xSrxMnO3. Using finite-temperature diagonalization techniques we investigate ate) for a model that contains only the e(g),-orbital degrees of freedom. Due to strong correlations the orbital model appears as a generalized t-J model with anisotropic interactions and three-site hopping. In the orbital t-J model sigma(omega) is characterized by a broad incoherent spectrum with increasing intensity as temperature is lowered, and a Drude peak with small weight, consistent with experiment. Our calculations for two-dimensional systems, which may have some particular relevance for the double-layer manganites, show that the scattering from orbital fluctuations can explain the order of magnitude of the incoherent part of sigma(omega) in the low-temperature ferromagnetic phase. Moreover, orbital correlation functions are studied and it is shown that x(2)-y(2) orbital order is preferred in the doped planar model at low temperature. [S0163-1829(99)01809-3].