  In iron-based superconductors, a unique tri-layer Fe-As (Se, Te, P) plays an
essential role in controlling the electronic properties, especially the Cooper
pairing interaction. Here we use scanning tunneling microscopy/spectroscopy
(STM/S) to investigate the role of arsenic atom in superconducting
Ba0.4K0.6Fe2As2 by directly breaking and restoring the Fe-As structure at
atomic scale. After the up-As-layer peeled away, the tunneling spectrum of the
exposed iron surface reveals a shallow incoherent gap, indicating a severe
suppression of superconductivity without arsenic covering. When a pair of
arsenic atoms is placed on such iron surface, a localized topographic feature
is formed due to Fe-As orbital hybridization, and the superconducting coherent
peaks recover locally with the gap magnitude the same as that on the iron-layer
fully covered by arsenic. These observations unravel the Fe-As interactions on
an atomic scale and imply its essential roles in the iron-based
superconductivity.
