EFFECTIVE RKKY INTERACTION IN HIGH-TC CUPRATES

We derive an effective spin Hamiltonian starting from the three-band m odel for the CuO2 planes in high temperature superconductors. The coup ling between the copper spins and the oxygen orbitals is eliminated us ing two subsequent canonical transformations. As a result, the copper spins interact via an ordinary superexchange and an effective RKKY int eraction, which increases with doping. The resulting frustration of ma gnetic interactions explains the suppression of antiferromagnetic long -range order upon doping.