2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/143556Electronic structure of stripe ordered La$_{2-x}$Sr$_{x}$NiO$_{4}$ is investigated. The system with x=1/3 is insulator, in LSDA+U calculations, and shows charge and spin stripe, consistent with the experimental results. Highly correlated system of x=1/2 is studied by using exact diagonalization of multi-orbital many body Hamiltonian derived from LDA calculations and including on-site and inter-site Coulomb interactions. The fluctuation of the residual spin on Ni$^{3+}$ (hole) site couples with the charge fluctuation between Ni$^{3+}$ and Ni$^{2+}$ states and this correlation lowers the total energy. The resultant ground state is insulator with charge and spin stripe of the energy gap 0.9eV, consistent with observed one. The on-site Coulomb interaction stabilizes integral valency of each Ni ion (Ni$^{3+}$ and Ni$^{2+}$), but does not induce the charge order. Two quantities, inter-site Coulomb interaction and anisotropy of hopping integrals, play an important role to form the charge and spin stripe order in a system of x=1/2.12 pages, 10 figures, fully revisedStrongly Correlated Electronstext