  The properties of iron-based superconductors (Fe-SCs) can be varied
dramatically with the introduction of dopants and atomic defects. As a pressing
example, FeSe, parent phase of the highest-$T_c$ Fe-SC, exhibits prevalent
defects with atomic-scale "dumbbell" signatures as imaged by scanning tunneling
microscopy (STM). These defects spoil superconductivity when their
concentration exceeds 2.5%. Resolving their chemical identity is prerequisite
to applications such as nanoscale patterning of
superconducting/nonsuperconducting regions in FeSe, as well as fundamental
questions such as the mechanism of superconductivity and the path by which the
defects destroy it. We use STM and density functional theory to characterize
and identify the dumbbell defects. In contrast to previous speculations about
Se adsorbates or substitutions, we find that an Fe-site vacancy is the most
energetically favorable defect in Se-rich conditions, and reproduces our
observed STM signature. Our calculations shed light more generally on the
nature of Se capping, the removal of Fe vacancies via annealing, and their
ordering into a $\sqrt{5}$$\times$$\sqrt{5}$ superstructure in FeSe and related
alkali-doped compounds.
