  A key actor in the conventional theory of superconductivity is the induced
interaction between electrons mediated by the exchange of virtual collective
fluctuations, originally phonons. Other collective modes that can play the same
role, especially spin-fluctuations, have been widely discussed in the context
of high-temperature and heavy Fermion superconductors. The strength of such
collective fluctuations is measured by the associated susceptibility. Here we
use differential elastoresistance measurements on five optimally doped Fe-based
superconductors to reveal that a diverging \emph{nematic} susceptibility
appears to be a generic feature in the optimal doping regime of these
materials. The observation motivates consideration of the effects of nematic
fluctuations on the superconducting pairing interaction in this family of
compounds, and possibly beyond.
