Orbital liquid state in ferromagnetic manganites

We discuss the role of orbital degeneracy in the optical conductivity of perovskite manganites in the ferromagnetic metallic phase. Employing an orbital t-J model we show that the anomalous features of the conductivity spectrum can be well accounted for assuming the existence of an orbital liquid state. We further extend our theory to include coupling to dynamical Jahn-Teller lattice modes, the result suggesting lattice effects to be present even in the metallic phase of manganites. The theory also is in accordance with the small values of T-linear specific heat found experimentally. (C) 1999 Elsevier Science B.V. All rights reserved.