Analysis of Resonant Inelastic X-ray Scattering in Stripe-Ordered Nickelate

We analyze theoretically the resonant inelastic x-ray scattering (RIXS) at the Ni K edge in the stripe-ordered state of La_2-xSr_xNiO₄ at x=1/3. In the calculation of RIXS spectra, the stripe-ordered ground state is described within the Hartree--Fock approximation by using a realistic tight-binding model for Ni 3dγ and O 2p_x,y orbitals, and the electron correlations in the electronic excitation processes are taken into account within the random-phase approximation. The calculated RIXS spectrum shows a tail toward the low-energy region when the momentum transfer of photons equals the stripe vector \mbi{Q}, being consistent with a recent experimental result. The origin of this anomalous momentum dependence of RIXS spectra is discussed microscopically.