Localization Effects of Cu Substitutions in FeSe: an Improved Green's Function Decoupling Approach

We study the substitution effects of Cu in FeSe system by using a Green's function decoupling method based on an extended Hubbard-I approximation. A three-band Hubbard Hamiltonian with randomly distributed impurities is employed to describe the iron superconductor Fe$_{1-x}$Cu$_{x}$Se. The parameters of the three-band model are determined by fitting the Fermi structures with the results of experiments. With the increasing of Cu substitution, the nesting between hole-like and electron-like Fermi pockets is destroyed completely. We also find that Cu substitution can introduce strong localization effects and lead the system to have a metal- insulator transition.