Excitations in the anisotropic spin-orbital model

We study the elementary excitations of the ferromagnetic spin-orbital model which represents the low-energy excitations for strongly correlated electrons described by the Hubbard Hamiltonian with a twofold degeneracy. At most two and four spin-and-orbital bound states have been found in one and two dimensions, respectively. For a completely degenerate system, the spin-and-orbital bound states are the lowest-lying elementary excitations in most of the Brillouin zone and for Ising-like interactions they can have much lower energy than the other excitations of the model and might thus be responsible for the instability of the ground state. This might lead to quantum disordered states in both spin and orbital space, as indicated by exact solutions obtained for small clusters.