Impurity-induced bound states inside the superconducting gap of FeSe

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 calculations on realistic slab models with an Fe vacancy, we show that such a defect does not induce a local magnetic moment. Therefore, the dumbbell defect is considered as nonmagnetic. Thus, the in-gap bound states emerging from a nonmagnetic defect-induced pair breaking suggest a sign-changing pairing state in this material.