SPIN AND ORBITAL EXCITATION SPECTRUM IN THE KUGEL-KHOMSKII MODEL

We discuss spin and orbital ordering in the twofold orbital degenerate superexchange model in three dimensions relevant to perovskite transi tion metal oxides. We focus on the particular point on the classical p hase diagram where orbital degeneracy is lifted by quantum effects exc lusively. Dispersion and damping of the spin and orbital excitations a re calculated at this point taking into account their mutual interacti on. Interaction corrections to the mean-field order parameters are fou nd to be small. We conclude that quasi-one-dimensional Neel spin order accompanied by the uniform d(3z2-r2)-type orbital ordering is stable against quantum fluctuations.