BROKEN SYMMETRY INSTABILITIES AND SOLUTIONS OF THE EXTENDED HUBBARD-MODEL

This report describes instabilities of the normal paramagnetic (RHF) state and broken symmetry solutions of the extended Hubbard model including the nearest neighbour repulsion and the exchange interaction on a planar square lattice. Based on the linear-response theory we derive instability conditions with ordering vector q = Q = (pi, pi) and q = Q(o) = (0,0) which have definite symmetry properties. For q = Q we obtain four non-magnetic states, a charge-density wave (CDW), a charge-current wave (CCW) and two bond-order waves (BOWs) and five magnetic states, an ordinary antiferromagnetism (AF), a spin-current wave (SCW), two axial-spin-bond-order waves (ASBOWs) and a helical-spin-bond-order wave (HSBOW). For q = Q(o) it is shown that there are three states, a BOW, a ferromagnetism (FM) and a spin-bond-order wave (SBOW).