LOW-ENERGY LIMIT OF THE 3-BAND MODEL FOR ELECTRONS IN A CUO2 PLANE

The three-band model with the O-O direct hopping near to unit filling is considered. We present the general procedure of reduction of this model to the low-energy limit. At unit filling the three-band model in the charge-transfer limit is reduced to the Heisenberg model and we calculate the superexchange constant. For the case of small electron doping the three-band model is reduced to the t-J model and we calculate electron hopping parameters at the nearest and next neighbors. We derive the structure of corrections to the t-J model and calculate their magnitudes. The values of the hopping parameters for electron-and hole-doping differ by approximately 40%.