Effect of transition-metal substitution in iron-based superconductors

We study theoretically the current debatable issue about the effect of transition-metal (TM) substitution in iron-based superconductors through treating all of the TM ions as randomly distributed impurities. The extra electrons from TM elements are localized at the impurity sites. In the mean time the chemical potential shifts upon substitution. The phase diagram is mapped out and it seems that the TM elements can act as effective dopants. The local density of states (LDOS) is calculated and the bottom becomes V-shaped as the impurity concentration increases. The LDOS at the Fermi energy $\rho(\omega=0)$ is finite and reaches the minimum at the optimal doping level. Our results are in good agreement with the scanning tunneling microscopy experiments.