Momentum-resolved electronic excitations in the Mott insulator Sr2IrO4 studied by resonant inelastic x-ray scattering

We report an Ir L-3 edge resonant inelastic x-ray scattering measurement of the low-lying electronic excitations in Sr2IrO4 over the complete Brillouin zone of the IrO2 plane. A remarkably strong inelastic signal which exceeds the elastic scattering in intensity is observed. Peaks observed at 0.5, 3.2, and 6.0 eV are respectively ascribed to an interband transition across the Mott gap and charge-transfer excitations from the O 2p band to the Ir 5d bands. The dispersion of the Mott gap excitation is found to be particularly weak. This is interpreted as a signature of the narrow 5d bands of the novel Mott insulating state of Sr2IrO4 induced by the strong spin-orbit interaction.