Resonant inelastic x-ray scattering study of electronic excitations in insulating K$_{0.83}$Fe$_{1.53}$Se$_2$

We report an Fe $K$-edge resonant inelastic X-ray scattering (RIXS) study of K$_{0.83}$Fe$_{1.53}$Se$_2$. This material is an insulator, unlike many parent compounds of iron-based superconductors. We found a sharp excitation around 1 eV, which is resonantly enhanced when the incident photon energy is tuned near the pre-edge region of the absorption spectrum. The spectral weight and line shape of this excitation exhibit clear momentum dependence. In addition, we observe momentum-independent broad interband transitions at higher excitation energy of 3-7 eV. Calculations based on a 70 band $dp$ orbital model, using a moderate $U_{rm eff}approx 2.5$ eV, indicate that the $sim$1 eV feature originates from the correlated Fe 3$d$ electrons, with a dominant $d_{xz}$ and $d_{yz}$ orbital character. We find that a moderate $U_{rm eff}$ yields a satisfying agreement with the experimental spectra, suggesting that the electron correlations in the insulating and metallic iron based superconductors are comparable.

Charge transfer in FeOCl intercalation compounds and its pressure dependence: An x-ray spectroscopic study

We present a study of charge transfer in Na-intercalated FeOCl and polyaniline-intercalated FeOCl using high-resolution x-ray absorption spectroscopy and resonant x-ray emission spectroscopy at the Fe-K edge. By comparing the experimental data with ab-initio simulations, we are able to unambiguously distinguish the spectral changes which appear due to intercalation into those of electronic origin and those of structural origin. For both systems, we find that about 25% of the Fe sites are reduced to Fe2+ via charge transfer between FeOCl and the intercalate. This is about twice as large as the Fe2+ fraction reported in studies using Mossbauer spectroscopy. This discrepancy is ascribed to the fact that the charge transfer occurs on the same time scale as the Mossbauer effect itself. Our result suggests that every intercalated atom or molecule is involved in the charge-transfer process, thus making this process a prerequisite for intercalation. The Fe2+ fraction is found to increase with pressure for polyaniline-FeOCl, hinting at an enhancement of the conductivity in the FeOCl intercalation compounds under pressure.