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.