On the electronic structure of electron doped LaOFeAs as seen by X-ray   absorption spectroscopy
  We investigated the recently found superconductor LaO_{1-x}F_xFeAs by X-ray absorption spectroscopy (XAS). From a comparison of the O K-edge with LDA calculations we find good agreement and are able to explain the structure and changes of the spectra with electron doping. An important result from this edge is a limitation of the Hubbard U to values not significantly larger than 1 eV.   From experimental Fe L_2,3-edge spectra and charge transfer multiplet calculations we gain further information on important physical values such as hopping parameters, the charge transfer energy Delta, and the on-site Hubbard U. Furthermore we find the system to be very covalent with a large amount of ligand holes. A shift in the chemical potential is visible in the O K- and Fe L_2,3-edge spectra which emphasizes the importance of band effects in these compounds. 
T. Kroll, S. Bonhommeau, T. Kachel, H.A. Duerr, J. Werner, G. Behr,   A.Koitzsch, R. Huebel, S. Leger, R. Schoenfelder, A. A...