Localized nature of valence-hole states in Ca_2+xY _2-xCu₅O₁₀ shown by O 1s XAS

The hole doping effects on the O 1s x-ray absorption spectrum (XAS) of a quasi-one-dimensional charge-transfer insulator, Ca_2+xY _2-xCu₅O₁₀, are discussed on the basis of numerically-exact diagonalization calculations applied to finite-size cluster models. The angle-resolved photoemission and inverse one are also calculated to understand the characteristic features in the XAS. The role of "edge-share"-type crystal structure, as well as that of electron correlation, is emphasized to understand the localized nature of the electronic states. The dp model Hamiltonian used in the O 1s XAS calculation can be mapped onto a single-band Hubbard model with the intra-atomic Coulomb repulsion, U, and the first and second neighbor hoppings, t₁ and t₂ (t ₁t₂).