Elementary excitations in the coupled spin-orbital model

The elementary excitations of a model Hamiltonian that captures the low-energy behavior of a simple twofold-degenerate Hubbard Hamiltonian, with Hund's rule coupling, is studied. The phase diagram in the mean-field 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. We show that, besides the usual spin waves (magnons), there also exist orbital waves (orbitons) and, most interestingly, in a completely ferromagnetically coupled system, a combined spin-orbital excitation which can be visualized as a bound state of magnons and orbitons. For a completely 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 as an example that elementary excitations in orbitally degenerate strongly correlated electron systems in general carry both spin and orbital character.