The Ordering of Polarons in the Holstein–t-J Model: An Application to La_2-xSr_xNiO_4+y

The recently observed ordering of polarons in La2-xSrxNiO4+y is investigated in terms of a simple one-band model of strongly correlated electrons at quarter filling. The lattice distorsions are described by an in-plane breathing mode which is treated in the adiabatic limit. We use exact diagonalization techniques on small lattices to obtain the ground-state configuration and the excitation spectrum of the Holstein-t-J model. For large enough electron-phonon coupling a state resembling the observed ordering of polarons appears as the ground state of the Holstein-t-J model. We discuss the dependence of the charge and magnetic structure factors on doping away from quarter filling. Within the limits of a finite-lattice calculation our results agree qualitatively with the experiment.