ORBITAL LIQUID IN PEROVSKITE TRANSITION-METAL OXIDES

We study the effects of the degeneracy of the e(g) orbitals in perovsk ite transition-metal oxides in the limit of strong repulsive electron- electron interaction. The isospin field is introduced to describe the orbital degrees of freedom and is represented by the boson with a cons traint. The dispersion of this boson is flat along (pi/a, pi/a, k(z)) (a: lattice constant) and the other two equivalent directions. This en ables the orbital disordered phase down to low temperatures. Some of t he anomalous experiments in the low-temperature ferromagnetic phase of La1-xSrxMnO3 are interpreted in terms of this orbital liquid picture. The coupling between the orbital degrees of freedom and the Jahn-Tell er lattice distortion and its implications to the optical conductivity are also discussed.