Intrinsic charge transfer gap in NiO from Ni K-edge x-ray absorption spectroscopy

Pre-edge features in x-ray absorption spectroscopy contain key information about the lowest excited states and thus on the most interesting physical properties of the system. In transition-metal oxides they are particularly structured but extracting physical parameters by comparison with a calculation is not easy due to several computational challenges. By combining core-hole attraction and correlation effects in a first-principles approach, we calculate Ni K-edge x-ray absorption spectra in NiO. We obtain a striking parameter-free agreement with experimental data and show that dipolar pre-edge features above the correlation gap are due to nonlocal excitations largely unaffected by the core hole. We show that in charge transfer insulators, this property can be used to measure the correlation gap and probe the intrinsic position of the upper Hubbard band.