  In conventional s-wave superconductors, only magnetic impurities exhibit
impurity bound states, whereas for an s+- order parameter they can occur for
both magnetic and non-magnetic impurities. Impurity bound states in
superconductors can thus provide important insight into the order parameter.
Here, we present a combined experimental and theoretical study of native and
engineered iron-site defects in LiFeAs. Detailed comparison of tunneling
spectra measured on impurities with spin fluctuation theory reveals a
continuous evolution from negligible impurity bound state features for weaker
scattering potential to clearly detectable states for somewhat stronger
scattering potentials. All bound states for these intermediate strength
potentials are pinned at or close to the gap edge of the smaller gap, a
phenomenon that we explain and ascribe to multi-orbital physics.
