Ordered valence bond states in symmetric two-dimensional spin-orbital systems

We consider a superexchange Hamiltonian, $H=-\sum_{<i,j>}(2{\bf S}_i\cdot {\bf S}_j-\frac 12)(2{\bf T}_i\cdot {\bf T}_j-\frac 12)$, which describes systems with orbital degeneracy and strong electron-phonon coupling in the limit of large on-site repulsion. In an SU(4) Schwinger boson representation, a reduced spin-orbital interaction is derived {\it exactly}, and a mean field theory has been developed by introducing a symmetric valence bond pairing order parameter. In one dimension, a spin-orbital liquid state with a finite gap is obtained. On a two-dimensional square lattice a novel type of spin-orbital ferromagnetically ordered state appears, while spin and orbital are antiferromagnetic. Moreover, an important relation has been found, relating the spin and orbital correlation functions to the combined spin-orbital ones.