Ubiquitous signatures of nematic quantum criticality in optimally doped Fe-based superconductors

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.