Interplay of charge and spin correlations in nickel perovskites

Analyzing the motion of low-spin (s=1/2) holes in a high-spin (S=1) background, we derive a sort of generalized t-J Hamiltonian for the NiO2 planes of Sr-doped nickelates. In addition to the rather complex carrier-spin and spin-spin couplings we take into account the coupling of the doped holes to in-plane oxygen breathing modes by a Holstein-type interaction term. Because of strong magnetic confinement effects the holes are nearly entirely prelocalized and the electron-phonon coupling becomes much more effective in forming polarons than in the isostructural cuprates. In the light of recent experiments on La2−xSrxNiO4 we discuss how the variety of the observed transport and charge/spin-ordering phenomena can be qualitatively understood in terms of our model Hamiltonian.