Optical conductivity of doped manganites: Comparison of ferromagnetic Kondo Lattice Models with and without orbital degeneracy

The optical conductivity of the double exchange and the orbital degenerate Kondo lattice Hamiltonian (orbital t-J model) are invetigated by means of finite temperature diagonalization for different doping concentrations. The temperature dependence of sum rules, kinetic energy and Drude weight are studied. In particular it is shown that the double exchange model cannot explain the anomalous absorption in the uniform ferromagnetic phase of manganites discovered Okimoto et al. [Phys. Rev. Lett. 75, 109 (1995)]. The orbital t-J model explains the increase of the anomalous absorption with decreasing temperature, its energy scale (~1 eV) and intensity. It also explains the appearance of the additional narrow Drude peak at low temperature.