Magnetic disorder and gap symmetry in optimally electron doped Sr(Fe, Co)₂As₂ superconductor

We investigate the magnetic pair-breaking due to Mn impurities in the optimally electron doped Sr(Fe$_{0.88} $Co$_{0.12} $)$_{2} $As$_2 $ superconductor to deduce the symmetry of the superconducting order parameter. Experiments on the as-grown crystals reveal a T$_c $ suppression rate of $ \sim $30 mK/$\mu \Omega cm$, which is in close agreement with similarly slower values of T$_c $ suppression rates reported previously for various transition metal impurities, both, magnetic and non-magnetic, in several structurally analogous iron-based superconductors. However, careful annealing of these crystals at low temperature for longer durations reveals new information crucial to the determination of the pairing symmetry. We found that the crystallographic defects are a significant source of pair-breaking in the as-grown crystals. We first establish that these defects are point-like by showing that their sole effect on electrical transport is to add a temperature independent scattering term that shifts the whole $ \rho $ vs. T curves rigidly up. The T$_c $ suppression rate due to these point-like defects is slow, $ \le$ 35 mK/$\mu \Omega cm$. On the other hand, T$_c $ suppression rate due to magnetic pair-breaking is estimated to be faster than 325 mK/$\mu \Omega cm$. A slower pair-breaking rate (measured in mK/$\mu \Omega cm$) than expected due to non-magnetic crystallographic defects, together with a faster pair-breaking rate due to magnetic impurities disfavors a sign-changing $s_{+-}$-wave and argues in the favor of a non-sign-changing $s_{++}$-wave state in the optimally electron doped SrFe$_2 $As$_2 $ superconductor.