Role of orbital degeneracy in double-exchange systems

We investigate the role of orbital degeneracy in the double-exchange model. In the ${J}_{H}\ensuremath{\rightarrow}\ensuremath{\infty}$ limit, an effective generalized ``Hubbard'' model incorporating orbital pseudospin degrees of freedom is derived. The model possesses an exact solution in one and in infinite dimensions. In one-dimension, the metallic phase off ``half-filling'' is a Luttinger liquid with pseudospin-charge separation. Using the $d=\ensuremath{\infty}$ solution for our effective model, we show how many experimental observations for the well-doped $(x\ensuremath{\simeq}0.3)$ three-dimensional manganites ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ can be qualitatively explained by invoking the role of orbital degeneracy in the double exchange model.