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. 
H. Gretarsson, T. Nomura, I. Jarrige, A. Lupascu, M. H. Upton, Jungho   Kim, D. Casa, T. Gog, R. H. Yuan, Z. G. Chen, N.-L. ...
cond-mat.supr-con