MEAN-FIELD PHASE-DIAGRAM OF A 2-BAND T-J MODEL FOR CUO2 LAYERS

We study the mean-field phase diagram of a two-band t-J model. Upon varying the doping and the values of the parameters, we identify solutions with different magnetic structure without spin long-range order. A first-order transition occurs between a uniform (rotationally-invariant) and a dimerized (non-rotationally-invariant) Fermi-liquid phase. A second transition (of the second order for low doping and of the first order for larger doping) takes place betwen the dimerized Fermi liquid and an insulating fully dimerized phase. We also find an instability region of negative compressibility where a phase separation occurs. At zero doping we find a Slater-to-charge-transfer-insulator transition where the compressibility vanishes together with the band dispersion. The results agree with those obtained in the J = 0 case and in the single-band Hubbard model at the Brinkman-Rice transition.