title: Ordered valence-bond states in symmetric two-dimensional spin-orbital systems.
abstract: We consider a superexchange Hamiltonian, H = -SUM (<i,j>)(2S(i) . S(j)-(1/2)) (2T(i) . T(j)-(1/2)), 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 exactly, and a mean field theory has been developed. 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. An important relation has been found, relating the spin and orbital correlation functions to the combined spin-orbital ones.
authors.affiliation: Department of Physics, the University of Hong Kong, Pokfulam Road, Hong Kong, China.
authors.forename: G M
authors.initials: GM
authors.lastname: Zhang
authors.forename: S Q
authors.initials: SQ
authors.lastname: Shen