Resonant inelastic X-ray scattering spectra at the Ir L-edge in Na2IrO3

We analyze resonant X-ray scattering (RIXS) spectra in Na2IrO3 on the basis of the itinerant electron picture. Employing a multi-orbital tight-binding model on a honeycomb lattice, we find that the zigzag magnetic order is the most stable with sizable energy gap in the one-electron band within the Hartree-Fock approximation. We derive the RIXS spectra, which are connected to the generalized density-density correlation function. We calculate the spectra as a function of excitation energy ω, within the random phase approximation. The spectra consist of the peaks with ω < 20 meV, and of the peaks with 0.4 < ω < 0.8 eV. The former peaks are composed of four bound states in the density-density correlation function, and may be identified as the magnetic excitations, while the latter peaks are composed of 16 bound states below the energy continuum of individual electron-hole pair excitations, and may be identified as the excitonic excitations. The calculated spectra agree qualitatively with the recent RIXS experiment.