  The pairing mechanism in the iron pnictides remains unresolved yet. One of
the central issues is the structure of the superconducting order parameter
which classifies the community into two different and highly disputed camps. On
one hand the picture of pairing based on the magnetic origin predicts a sign
reversal gap on the electron and hole Fermi pockets, leading to the S+-
pairing. On the other hand, a more conventional S++ pairing gap was suggested
based on the phonon or orbital fluctuation mediated pairing. In the
superconducting state, the impurities may generate a unique pattern of local
density of states in space and energy, which are regarded as the fingerprints
for checking the structure of the pairing gap. In this study, we successfully
identified the non-magnetic and magnetic impurities in Na(Fe0.97-xCo0.03Tx)As
(T=Cu, Mn) and investigated the spatial resolved scanning tunneling
spectroscopy. We present clear evidence of the in-gap quasiparticle states
induced by the nonmagnetic Cu impurities, giving decisive evidence of the S+-
pairing. This is corroborated by the consistency between the experimental data
and the first-principles calculations based on the S+- pairing gap with a
scalar scattering potential.
