CONSISTENT LOW-ENERGY REDUCTION OF THE 3-BAND MODEL FOR ELECTRONS ANDHOLES IN COPPER OXIDES TO THE EFFECTIVE T-J MODEL

A full three-band model for the CuO2 plane with inclusion of all essen tial interactions - Cu-O and O-O hopping, repulsion at the copper and oxygen and between them - is considered. A general procedure of the lo w-energy reduction of the primary Hamiltonian to the Hamiltonian for h oles and electrons is developed. For the purpose of fixing of the t-J model parameters we calculate the values of the superexchange constant J and the charge-transfer gap E(gap) in the framework of the three-ba nd model. Fitting values of J and E(gap) to the experimental data allo ws to narrow the uncertainty region of the three-band model parameters . As a result the ratio t/J of the t-J model is fixed in the range 2.4 divided by 2.7 for holes and 2.5 divided by 3.0 for electrons.