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/spectroscopy on the A-122 iron pnictide superconductors - in this case BaFe$_{2-x}$Co$_x$As$_2$ - involve a strong adatom filtering of the differential conductance from the near-EF Fe3d states, which in turn originates from the top-most sub-surface 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 an energy-dependent contrast inversion. 
K. Koepernik, S. Johnston, E. van Heumen, Y. Huang, J. Kaas, J. B.   Goedkoop, M. S. Golden, and Jeroen van den Brink
cond-mat.supr-con