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

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.PACS numbers:

X-ray absorption study of the local structure at the NiO/oxide interfaces

“…However, the EXAFS-obtained spectra did not have sufficient quality for EXAFS analysis since the signal-to-noise ratio was low owing to the very small amount of deposited material needed for the early stages study. The same applies to the occurrence of pre-edge structures, commonly addressed to off-site transitions, whose origin is presently under debate (Kuzmin et al, 1995;Vedrinskii et al, 2001;Gougoussis et al, 2009). These drawbacks were overcome by studying the X-ray absorption near-edge structure (XANES) region of the spectra which exhibits, in all cases, sufficient quality to perform the aforementioned structural characterization of the NiO/oxide systems.…”

Multiple pre-edge structures in CuK-edge x-ray absorption spectra of high-Tccuprates revealed by high-resolution x-ray absorption spectroscopy

“…When excitations due to intersite Cu (absorbing atom) 4p -Cu (other atom) 3d hybridization occur, the information is very relevant since these features are typically weakly affected by core-hole attraction and thus they probe the excitation in the absence of a core-hole in the final state. In some cases off-site transitions can be used to measure the charge transfer gap, as was recently suggested for NiO [2]. Therefore, a full understanding of pre-edge features and of their possible non-local nature leads to the knowledge of the hybridization mechanism between different orbitals in the low energy region (up to some eV from the Fermi level) and of the position of the upper Hubbard band in the absence of a core-hole in the final state [2].…”

“…In some cases off-site transitions can be used to measure the charge transfer gap, as was recently suggested for NiO [2]. Therefore, a full understanding of pre-edge features and of their possible non-local nature leads to the knowledge of the hybridization mechanism between different orbitals in the low energy region (up to some eV from the Fermi level) and of the position of the upper Hubbard band in the absence of a core-hole in the final state [2].…”

“…Understanding the nature of the pre-edge features is relevant since these excitations are low in energy and probe the local environment of the absorbing atom. In several recent works [1][2][3][4] the presence of off-site excitations in transition metal compounds like NiO, cuprates and cobaltates was demonstrated. Off-site excitations can occur by a direct transition of a 1s core electron to empty electronic states of the nearest neighboring atoms [2].…”

“…In several recent works [1][2][3][4] the presence of off-site excitations in transition metal compounds like NiO, cuprates and cobaltates was demonstrated. Off-site excitations can occur by a direct transition of a 1s core electron to empty electronic states of the nearest neighboring atoms [2]. This mechanism has a low intensity (∼ 1% of the edge jump) since the p-states of the absorbing atoms are not involved.…”