Unexpected weak spatial variation in the local density of states induced by individual Co impurity atoms in superconducting Na(Fe$_1$-$x$Co$_x$)As crystals revealed by scanning tunneling spectroscopy

We use spatially resolved scanning tunneling spectroscopy in Na(Fe1âxCox)As to investigate the impurity effect induced by Co dopants. The Co impurities are successfully identified, and the spatial distributions of local density of state at different energies around these impurities are investigated. It is found that the spectrum shows negligible spatial variation at different positions near the Co impurity, although there is a continuum of the in-gap states which lifts the zero-bias conductance to a finite value. Our results put constraints on the S± and S++ models and sharpen the debate on the role of scattering potentials induced by the Co dopants.