Effects of cobalt doping and three-dimensionality in BaFe₂As₂

We investigate the dual roles of a cobalt impurity in the Ba-122 ferropnictide superconductor in the state with coexisting collinear spin-density wave (SDW) order as a dopant and as a scattering center, using first-principles electronic-structure methods. The Co atom is found to dope the FeAs plane where it is located with a single delocalized electron as expected, but also induces a strong perturbation of the SDW ground state of the system. This in turn induces a stripelike modulation of the density of states in nearby planes which may be observable in scanning tunnel microscope (STM) experiments. The defect is found to have an intermediate strength nonmagnetic scattering potential with a range of roughly 1 Å, and the Co gives rise to a smaller but longer range magnetic scattering potential. The impurity potential in both channels is highly anisotropic, reflecting the broken symmetry of the SDW ground state. We give values for the effective Co potentials for each d orbital on the impurity and nearby sites. The calculation also shows a clear local resonance comprised of Co states about 200 meV above the Fermi level, in quantitative agreement with a recent report from STM. Finally, we discuss the issue of the effective dimensionality of the 122 materials, and show that the hybridization of the out-of-phase As atoms leads to a higher density of states between the FeAs planes relative to the 1111 counterparts.