Landau theory of stripe phases in cuprates and nickelates

We consider a Landau theory of coupled charge and spin-density-wave order parameters as a simple model for the ordering that has been observed experimentally in the La${}_2$NiO${}_4$ and La${}_2$CuO${}_4$ families of doped antiferromagnets. The period of the charge-density wave is generically half that of the spin-density wave, or equivalently the charges form antiphase domain walls in the antiferromagnetic order. A sharp distinction exists between the case in which the ordering is primarily charge driven (which produces a sequence of transitions in qualitative agreement with experiment) or spin driven (which does not). We also find that stripes with noncollinear spin order (i.e., spiral phases) are possible in a region of the phase diagram where the transition is spin driven; the spiral is circular only when there is no charge order, and is otherwise elliptical with an eccentricity proportional to the magnitude of the charge order.