Character of doped oxygen holes in high-TcCu oxides

The symmetry and site distribution of doped oxygen holes are studied using the $\mathrm{Cu}{\mathrm{O}}_n$ cluster model. For reasonable values of the on-site Coulomb interaction, charge transfer energy, and hybridization, the holes are found to enter $p{\sigma }_{x,y}$ orbitals in the Cu${\mathrm{O}}_2$ plane and to form local singlets with the Cu $d^9$ spins when the energy difference between the in-plane and off-plane (BaO) oxygen levels is small, whereas the holes are distributed both in the Cu${\mathrm{O}}_2$ and BaO planes and form local triplets when the BaO oxygen energy level is high enough (by ≳ 1.7 eV). We argue that the latter situation may occur in high-$T_c$ superconductors and may be related to the occurrence of superconductivity.