  We consider a localized impurity atom that interacts with a cloud of fermions
in the paired state. We develop an effective scattering length description of
the interaction between an impurity and a fermionic atom using their vacuum
scattering length. Treating the pairing of fermions at the mean-field level, we
show that the impurity atom acts like a magnetic impurity in the condensed
matter context, and leads to the formation of a pair of Shiba bound states
inside the superconducting gap. In addition, the impurity atom can lead to the
formation of deeply bound states below the Fermi sea.
