ELEMENTARY EXCITATIONS IN THE COUPLED SPIN-ORBITAL MODEL

The elementary excitations of a model Hamiltonian that captures the lo w-energy behavior of a simple twofold-degenerate Hubbard Hamiltonian, with Hund's rule coupling, is studied. The phase diagram in the mean-f ield limit and in a two-site approach reveals a rich variety of phases where both the orbital and the spin degrees of freedom are ordered. W e show that, besides the usual spin waves (magnons), there also exist orbital waves (orbitons) and, most interestingly, in a completely ferr omagnetically coupled system, a combined spin-orbital excitation which can be visualized as a bound state of magnons and orbitons. For a com pletely degenerate system the bound states are found to be the lowest- lying elementary excitations, both in one and two dimensions. Finally we extend our treatment to almost-degenerate systems. This can serve a s an example that elementary excitations in orbitally degenerate stron gly correlated electron systems in general carry both spin and orbital character. [S0163-1829(98)10839-1].