Orbital dynamics in ferromagnetic transition-metal oxides

We consider a model of strongly correlated e(g) electrons interacting by su perexchange orbital interactions in the ferromagnetic phase of LaMnO3. It i s found that the classical orbital order with alternating occupied e(g) orb itals has a full rotational symmetry at orbital degeneracy, and the excitat ion spectrum derived using the linear spin-wave theory is gapless. The quan tum corrections to the order parameter and to the ground state energy resto re the cubic symmetry of the model. By applying a uniaxial pressure orbital degeneracy is lifted in a tetragonal field and one finds an orbital-flop p hase with a gap in the excitation spectrum. In two dimensions the classical order is more robust near the orbital degeneracy and quantum effects are s uppressed. The orbital excitations obtained using finite temperature diagon alization of two-dimensional clusters consist of a quasiparticle accompanie d by satellite structures. The orbital waves found within the linear spin-w ave theory provide an excellent description of the dominant pole of these s pectra. [S0163-1829(99)05309-6].