Significance of the inter-site Coulomb interaction between the O 2p and Cu 3d holes revealed by resonant inelastic x-ray scattering of Sr ₁₄Cu₂₄O₄₁

We have carried out resonant inelastic x-ray scattering (RIXS) measurements at the Cu K-edge on Sr₁₄Cu₂₄O₄₁, which has two kinds of layers, namely, the ladder layer and the chain layer. The excitation from the Zhang-Rice singlet band to the upper Hubbard band (UHB) shows a large dispersion with a periodicity of the ladder layer. The excitation corresponding to the ligand-to-metal charge transfer is nearly dispersionless. Comparison between the RIXS data and theoretical calculations shows that the inter-site Coulomb interaction between the Cu 3d and O 2p holes plays a prominent role in the RIXS process. This interaction is expected to be important in the hole state in the ladder layer of this compound.