  The quasiparticle states around a nonmagnetic impurity in electron-doped
iron-based superconductors with spin-density-wave (SDW) order are investigated
as a function of doping and impurity scattering strength. In the undoped
sample, where a pure SDW state exists, two impurity-induced resonance peaks are
observed around the impurity site and they are shifted to higher (lower)
energies as the strength of the positive (negative) scattering potential (SP)
is increased. For the doped samples where the SDW order and the superconducting
order coexist, the main feature is the existence of sharp in-gap resonance
peaks whose positions and intensity depend on the strength of the SP and the
doping concentration. In all cases, the local density of states exhibits clear
$C_2$ symmetry. We also note that in the doped cases, the impurity will divide
the system into two sublattices with distinct values of magnetic order. Here we
use the band structure of a two-orbital model, which considers the asymmetry of
the As atoms above and below the Fe-Fe plane. This model is suitable to study
the properties of the surface layers in the iron-pnictides and should be more
appropriate to describe the scanning tunneling microscopy experiments.
