Spin and orbital excitations in magnetic insulators with Jahn-Teller ions

The elementary excitations of a model Hamiltonian that captures the low energy behavior of a simple two-fold 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 usual spin waves (magnons) there exist also 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 the elementary excitations in orbitally degenerate strongly correlated electron systems in general carry both spin and orbital character.