SYMMETRIES AND MEAN-FIELD PHASES OF THE EXTENDED HUBBARD-MODEL

The two-dimensional extended Hubbard model that includes a nearest-nei ghbor Heisenberg interaction is studied using a mean-held theory where quasiparticles are defined by an U(8) group of canonical transformati ons permitting both broken gauge, spin, and sublattice symmetry. The t heory is further extended to incorporate a possible twist in the spin- quantization axis, so that the competition between superconductivity, charge-density waves, and Neel and spiral antiferromagnetic order can be monitored within one single theory. Our results for positive Hubbar d U and Heisenberg exchange J suggest that antiferromagnetic ordering dominates close to half-filling, while spiral states and d-wave superc onductivity compete when doping is introduced. For moderate values of J, we find a phase diagram where a phase transition occurs from an ant iferromagnet to a d-wave superconductor as doping is increased. A narr ow region of (s + id)-wave superconductor is found for some values of J and U.