Strong Similarities between the Local Electronic Structure of Insulating Iron Pnictide and Lightly Doped Cuprate

One of the major puzzles regarding unconventional superconductivity is how some of the most interesting superconductors are related to an insulating phase that lies in close proximity. Here, we report scanning tunneling microscopy studies of the local electronic structure of Cu-doped NaFeAs across the superconductor-to-insulator transition. We find that in the highly insulating regime, the electronic spectrum develops an energy gap with diminishing density of states at the Fermi level. The overall line shape and strong spatial variations of the spectra are strikingly similar to those of lightly doped cuprates close to the parent Mott insulator. We propose that the suppression of the itinerant electron states and the strong impurity potential induced by Cu dopants lead to this insulating iron pnictide.