CRITERION FOR THE VALIDITY OF THE T-J MODEL BASED UPON A 2-BAND MODEL FOR COPPER OXIDES

The second-order effective Hamiltonian in the representation of a set of orthogonal operators is derived from a two-band model for Cu oxides. The contribution from the local Cu2+ and surrounding O- spin triplet state to various hopping processes has been included. From the fourth-order perturbation we study doping effects on the background spin-spin interaction and obtain the expression for the ferromagnetic superexchange interaction between two neighboring Cu2+ spins with an O- hole in the middle. The condition for the equivalence between the t-J model and the two-band model for Cu oxides is discussed. In terms of nonorthogonal operators for a single O- hole, the condition for the exact mapping of a two-band model to a single-band model has also been derived. © 1990 The American Physical Society.