Fermi-Suface Evolution by Transition-metal Substitution in the Iron-based Superconductor LaFeAsO

We study how Co- and Ni-substitution affect the electronic structure of the iron-based superconductor, LaFeAsO. We perform {\it ab initio} supercell calculations and unfold the first Brillouin zone (BZ) to calculate the spectral function in the BZ for the normal cell. The charge density distribution in real space shows that doped extra electrons are trapped around Co (Ni) atom. This seems to mean that Co(Ni)-substitution does not work as carrier doping. However, the present momentum-space analysis indicates that the Fermi-surface volume indeed expands by substitutions, which can be well described by the rigid-band shift. By taking into account this effective doping, we discuss whether the sign-reversing s-wave ($s_{\pm}$-wave) scenario is compatible with experiments.