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

The experimental transport scattering rate was determined for a wide range of optimally doped transition-metal-substituted FeAs-based compounds with the ThCr2Si2 (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 similar to 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 x 10(14) s(-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.