Drastic Effect of Impurity Scattering on the Electronic and Superconducting Properties of Cu-doped FeSe

Nonmagnetic impurities in iron-based superconductors can provide an important tool to understand the pair symmetry and they can significantly influence the transport and the superconducting behavior. Here, we present a study of the role of strong impurity potential in the Fe plane, induced by Cu substitution, on the electronic and superconducting properties of single crystals of FeSe. The addition of Cu quickly suppresses both the nematic and superconducting states and increases the residual resistivity due to enhanced impurity scattering. Using magnetotransport data up to 35 T for a small amount of Cu impurity, we detect a significant reduction in the mobility of the charge carriers by a factor of ???3. While the electronic conduction is strongly disrupted by Cu substitution, we identify additional signatures of anisotropic scattering which manifest in linear resistivity at low temperatures and H1.6 dependence of magnetoresistance. The suppression of superconductivity by Cu substitution is consistent with a sign-changing s?? order parameter. Additionally, in the presence of compressive strain, the superconductivity is enhanced, similar to FeSe.