Surface adatom conductance filtering in scanning tunneling spectroscopy of Co-doped BaFe₂As₂ iron pnictide superconductors

We establish in a combination of ab initio theory and experiments that the tunneling process in scanning tunneling microscopy or spectroscopy on the A-122 iron pnictide superconductors—in this case BaFe2-xCoxAs2—involves a strong adatom filtering of the differential conductance from the near-EF Fe-3d states, which in turn originates from the topmost subsurface Fe layer of the crystal. The calculations show that the dominance of surface Ba-related tunneling pathways leaves fingerprints found in the experimental differential conductance data, including large particle-hole asymmetry and energy-dependent contrast inversion in conductance maps.