Oxygen x-ray emission and absorption spectra as a probe of the electronic structure of strongly correlated oxides

We present synchrotron-excited oxygen x-ray $K$-emission spectroscopy ($\text{O}K\alpha$ XES) and oxygen x-ray absorption spectroscopy ($\text{O}1s$ XAS) spectra of transition-metal (TM) oxides MnO, CoO, and NiO. The comparison of oxygen $K$-emission and absorption spectra to valence band photoemission and bremsstrahlung isochromat spectra measurements shows that $\text{O}1s$ XAS is not strongly influenced by the core hole effect, whereas the $\text{TM}2p$ XAS significantly shifts to a lower energy. New and effective methods for determining the band gap and anion-to-cation cation charge-transfer energies of the oxides from the measured spectra are presented and applied, and the combination of O XAS and XES is shown to agree well with the results of numerical electronic structure methods applied to strongly correlated oxides. For MnO, the charge-transfer energy is found to be 6.6 eV and the band gap is 4.1 eV; for CoO, the values are 6.1 and 2.6 eV and for NiO, the values are 5.4 and 4.0 eV.

Figure 1

The comparison of oxygen $K$- and Ni $L$-emission and absorption spectra to XPS VB and BIS of NiO.

Figure 2

The comparison of oxygen $K$- and Mn $L$-emission and absorption spectra to XPS VB and BIS of MnO.

Figure 3

The comparison of oxygen $K$- and Co $L$-emission and absorption spectra to XPS VB and BIS of CoO.

Figure 4

The estimation of band gap for NiO, CoO, and MnO from oxygen $K$-emission and absorption spectra.

Figure 5

The comparison of $\text{O}2p$ DOS of NiO calculated in GGA (Ref. 20), $\text{LDA}+U$ (Ref. 20), and $\text{LDA}+\text{DMFT}\left(\text{QMC}\right)$ (Ref. 21) approaches to $\text{O}K\alpha$ XES and $\text{O}1s$ XAS of NiO.