A direct probe of the variability of Coulomb correlation in Fe-pnictide   superconductors
  We use core-valence-valence (CVV) Auger spectra to probe the Coulomb repulsion between holes in the valence band of Fe pnictide superconductors. By comparing the two-hole final state spectra to density functional theory calculations of the single particle density of states, we extract a measure of the electron correlations that exist in these systems. Our results show that the Coulomb repulsion is highly screened and can definitively be considered as weak. We also find that there are differences between the 1111 and 122 families and even a small variation as a function of the doping, x, in Ba(Fe1 xCox)2As2. We discuss how the values of the hole-hole Coulomb repulsion obtained from our study relate to the onsite Coulomb parameter "U" used in model and first principles calculations based on dynamical mean field theory, and establish an upper bound for its effective value. Our results impose stringent constraints on model based phase diagrams 
P. Vilmercati, C. Parks Cheney, F. Bondino, E. Magnano, M. Malvestuto,   M.A. McGuire, A.S. Sefat, B.C. Sales, D. Mandrus, D...
cond-mat.supr-con cond-mat.str-el