  We investigate the local density of states in the vicinity of a native
dumbbell defect arising from an Fe vacancy in FeSe single crystals. The
tunneling spectra close to the impurity display two bound states inside the
superconducting gap, equally spaced with respect to zero energy but asymmetric
in amplitude. Using spin-polarized density functional theory (DFT) calculations
on realistic slab models with Fe vacancy, we show that such a defect does not
induce a local magnetic moment. Therefore, the dumbbell defect is considered as
non-magnetic. Thus, the in-gap bound states emerging from a non-magnetic
defect-induced pair-breaking suggest a sign changing pairing state in this
material.
