Large-cluster effects on core-level photoabsorption spectrum of LaCuO₃

In order to investiaget the influence of Cu-Cu interaction on the Cu 2p core-level x-ray absorption spectrum of LaCuO₃, the Lanczos calculations are performed for (Cu₃O₁₆)^23- and (CuO₁₆)^29- cluster models. The Cu 2p XAS is calculated as a function of the charge-transfer energy (Δ) between the Cu 3d and O 2p states. With decreasing Δ, the spin state of the ground state changes from magnetic to nonmagnetic. Around the critical Δ, the main line of the XAS splits by about 1.5 eV, reproducing the experimental result of LaCuO₃ well. In such low Δ cases, half the valence holes occupy the O 2p states, mediating the strong Cu-Cu coupling, and the photoabsorption at a particular Cu site inevitably affects the electronic states around the neighboring Cu sites, in contrast to divalent Cu compounds. This inter-site coupling is enhanced, when the tetragonal distortion of CuO₆ units is small. In other owrds, the 3d orbital degeneracy is important in discussing the Cu 2p XAS of LaCuO₃. The Cu 2p photoemission spectrum is also calculated.