Evidence for holes on oxygen sites in the high-$T_c$ superconductors ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_4$ and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{y}}$

The local oxygen density of states of p symmetry at the Fermi level of ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$ and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{y}}$ has been investigated as a function of x and y. Core-level excitations of oxygen 1s electrons into empty states by electron energy-loss spectroscopy has been used. Strong deviations from our calculations of the O 1s edges in the framework of a local density-functional model were observed in the superconducting and in the semiconducting compounds. They can be explained by taking into account on-site correlations of the Cu 3d electrons. The results indicate that for the superconductors, the density of states at the Fermi energy has predominantly oxygen 2p character. In this band the charge carriers in the superconducting compounds are holes. In the semiconducting compounds, the oxygen 2p band is probably completely filled.