Electronic structure of charge and spin stripe order inLa2−xSrxNiO4(x=13,12)

Abstract

Electronic structure of stripe ordered ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ni}{\mathrm{O}}_{4}$ is investigated. The system with $x=\frac{1}{3}$ is insulator, in $\mathrm{LSDA}+U$ calculations, and shows charge and spin stripe, consistent with the experimental results. A highly correlated system of $x=\frac{1}{2}$ is studied by using exact diagonalization of the multiorbital many-body Hamiltonian derived from LDA calculations and including on-site and intersite Coulomb interactions. The fluctuation of the residual spin on ${\mathrm{Ni}}^{3+}$ (hole) site couples with the charge fluctuation between ${\mathrm{Ni}}^{3+}$ and ${\mathrm{Ni}}^{2+}$ states and this correlation lowers the total energy. The resultant ground state is an insulator with charge and spin stripe of the energy gap $0.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, consistent with the observed one. The on-site Coulomb interaction stabilizes integral valency of each Ni ion (${\mathrm{Ni}}^{3+}$ and ${\mathrm{Ni}}^{2+}$), but does not induce the charge order. Two quantities, intersite Coulomb interaction and anisotropy of hopping integrals, play an important role to form the charge and spin stripe order in a system of $x=\frac{1}{2}$.