Effect of lattice dimerization on a system with coexisting charge and spindensity waves

An extended Hubbard-Holstein model is used to investigate the conditions fo r the existence and the stability of spin density wave antiferromagnetic or der in a system with a chemically induced charge density wave. in the prese nce of a lattice dimerization, produced by the coupling of the electron den sity to the lattice, in the adiabatic limit. In the weak coupling limit the antiferromagnetic ground state is slightly affected and can support a latt ice dimerization. As the coupling to the lattice is increased a metastable situation is reached in which the dimerized antiferromagnetic phase compete s with a paramagnetic phase with a larger Lattice dimerization. A first-ord er phase transition is found in the metastable region. The dimerized antife rromagnetic phase disappears abruptly at a critical value of the electron-l attice coupling.