BROKEN SYMMETRY SOLUTIONS OF THE 2-DIMENSIONAL EXTENDED HUBBARD-MODEL AND THEIR INSTABILITY CONDITIONS

lnstabilities of the restricted Hartree-Fock (HF) state of the two-dimensional extended Hubbard model under doping are studied with the linear response theory employing the linearized Bethe-Salpeter equation. Boundaries of the instabilities for the ordering vectors q = Q = (pi, pi) and q = Q0 = (0,0) are given in terms of the interaction strength parameters at various doping levels. With doping, dominant instabilities generally change from ones for q = Q to ones for q = Q0. Broken symmetry phases obtained from the instabilities are also studied and classified using the group-theoretical technique. For q = Q, two more new phases are obtained in addition to the four nonmagnetic and five magnetic phases that have been already derived by one of the authors. For q = Q0, three nonmagnetic and five magnetic phases are obtained. Deriving the explicit forms of the charge current and spin current operators in the HF approximation, charge current and spin current densities in the broken symmetry states are discussed in detail.