PHASE-SEPARATION AND 3-SITE HOPPING IN THE 2-DIMENSIONAL T-J MODEL

We study the t-J model with the inclusion of the so-called three-site term which is due to the t/U --> 0 expansion of the Hubbard model. We find that this singlet pair hopping term has no qualitative effect on the structure of the pure mean field phase diagram for non-magnetic st ates. In accordance with experimental data on high-T-c materials and s ome numerical studies, we also find wide regions of phase coexistence whenever the coupling J is greater than a critical value J(c). We show that J(c) varies linearly with the temperature T, going to zero at T = 0. (C) 1997 Elsevier Science B.V.