Measuring the intrinsic charge transfer gap using K-edge X-ray absorption spectroscopy

TL;DR

This paper demonstrates a first-principles method to accurately measure the charge transfer gap in transition metal oxides using Ni K-edge X-ray absorption spectroscopy, revealing insights into non-local excitations and the upper-Hubbard band.

Contribution

It introduces a parameter-free computational approach that combines core-hole effects and correlation to interpret pre-edge features in XAS for charge transfer insulators.

Findings

Achieves excellent agreement with experimental spectra for NiO.

Identifies non-local excitations as the origin of pre-edge features.

Provides a method to measure the correlation gap and upper-Hubbard band position.

Abstract

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 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 non-local 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.