Universal pair-breaking in transition-metal-substituted iron-pnictide superconductors

Abstract

The experimental transport scattering rate was determined for a wide range of optimally doped transition-metal-substituted FeAs-based compounds with the ThCr${}_{2}$Si${}_{2}$ (122) crystal structure. The maximum transition temperature ${T}_{c}$ for several Ba-, Sr-, and Ca-based 122 systems follows a universal rate of suppression with increasing scattering rate indicative of a common pair-breaking mechanism. Extraction of standard pair-breaking parameters puts a limit of $\ensuremath{\sim}$26 K on the maximum ${T}_{c}$ for all transition-metal-substituted 122 systems, in agreement with experimental observations, and sets a critical scattering rate of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{14}$ s${}^{\ensuremath{-}1}$ for the suppression of the superconducting phase. The observed critical scattering rate is much weaker than that expected for a sign-changing order parameter with strong interband scattering, providing important constraints on the nature of the superconducting gap in the 122 family of iron-based superconductors.