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

Abstract

Recent optical conductivity $\ensuremath{\sigma}(\ensuremath{\omega})$ experiments have revealed an anomalous spectral distribution in the ferromagnetic phase of the perovskite system ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$. Using finite-temperature diagonalization techniques we investigate $\ensuremath{\sigma}(\ensuremath{\omega})$ 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 $\ensuremath{\sigma}(\ensuremath{\omega})$ 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 $\ensuremath{\sigma}(\ensuremath{\omega})$ 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.