Schrieffer‐Wolff Transformation of the Emery Model for Cu–O Superconductors

The minimal version of the Emery model – ascribed to a CuO 2 layer in high‐ T c superconductors – is subject to a canonical transformation of the Schrieffer‐Wolff type up to the fourth order in the hybridization amplitude. An effective Hamiltonian is extracted systematically consisting of three parts: the kinetic energy of O‐holes, a Kondo‐like Cu–O exchange interaction, and the Heisenbergtype superexchange interaction between localized Cu spins with a modified coupling parameter in the undoped case. The assumptions and approximations underlying the derivation of the t̃– J pd – J dd model are pointed out.