Direct observation of the on-site oxygen 2p two-hole Coulomb energy in La2CuO4.

Electron correlation in functional materials has remained a challenge with strong deviations of electronic structure from mean field approaches. In high temperature superconductors the electron-electron and hole-hole interaction energies are essential in the underlying pairing mechanisms. For cuprates, oxygen holes have been considered of central importance for superconductivity. In La2CuO4 the site specific oxygen 2p hole-hole Coulomb energy has been determined by Auger photoelectron coincidence spectroscopy. This experimental approach allows to separate the different oxygen sites, i.e. the lattice oxygen, and distinguish from otherwise overlapping signal from surface oxygen. Values of 6.3  ± 0.2 eV for oxygen in the Cu-O planes and an upper limit of 9.2  ± 0.2 eV for apical oxygen are found to be on the high energy side of reported computational values and narrows the range of experimentally reported values. Additionally, a much reduced hybridization in La2CuO4 as compared to CuO is found in O 2p hybridization strengths.