Alternating spin and orbital dimerization and spin-gap formation in coupled spin-orbital systems

We discuss a novel mechanism for obtaining a spin-gap state through the cre ation of a coherent spin-orbital structure in systems with orbital degenera cy. Using the density matrix renormalization group, we calculate the phase diagram for a coupled spin-orbital model. We find that, in addition to ferr omagnetic and power-law antiferromagnetic phases for spin and orbital degre es of freedom, this model has a gapless line extending from the ferromagnet ic phase to the Bethe ansatz solvable SU(4) critical point, and a gapped ph ase with doubly degenerate ground states which form alternating spin and or bital singlets. The relevance to several recently discovered materials is d iscussed.