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 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 $\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 $1.5\times {10}^{14}$ 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.

Figure 1

Phase diagrams of the SrFe${}_{2-x}$Ni${}_x$As${}_2$ (Ref. 9) and SrFe${}_{2-x}$Pt${}_x$As${}_2$ (Ref. 12) systems. Antiferromagnetic (triangles) and superconducting (diamonds) transition temperatures are plotted for Ni- (open symbols) and Pt-doped (closed symbols) systems. The similar rate of suppression of the magnetic phases and the position of the superconducting domes, with optimal doping at $x\simeq 0.16$ for both cases, is to be contrasted with the considerably different $T_{c\left(\max \right)}$ values of 9 K and 16 K for Ni- and Pt-doped series, respectively.

Figure 2

Six-wire measurements of resistivity (main panel) and Hall coefficient (inset) of optimally doped SrFe${}_{2-x}$Pd${}_x$As${}_2$, SrFe${}_{2-x}$Ni${}_x$As${}_2$, SrFe${}_{2-x}$Co${}_x$As${}_2$, and BaFe${}_{2-x}$Pt${}_x$As${}_2$, with $T_c$ values of 7, 8, 17, and 20 K, respectively. The schematic depicts the configuration of the six-wire measurement.

Figure 3

Effect of transition-metal substitution on $T_c$ values of a wide variety of 122 superconductors at optimal doping concentrations, plotted as a function of the experimental transport scattering rate $\Gamma \equiv e\rho /R_H{m}^{*}$ (see text). Closed symbols indicate six-wire measurements (see text), and open symbols indicate values obtained from literature data for (1) CaFe${}_{1.92}$Co${}_{0.08}$As${}_2$ (Ref. 45), (2) BaFe${}_{2-x}$Co${}_x$As${}_2$ (Refs. 19 and 46), (3) BaFe${}_{2-x}$Ni${}_x$As${}_2$ (Refs. 47 and 48), (4) SrFe${}_{1.74}$Co${}_{0.26}$As${}_2$ (Ref. 49), and (5) SrFe${}_{1.84}$Ni${}_{0.16}$As${}_2$ [${\rho }_0=212$ $\mu \Omega$ cm, $R_H$(0 K) = $1.4\times {10}^{-9}$ m${}^3$/C] (Ref. 50). Samples of SrFe${}_{2-x}$Co${}_x$As${}_2$ and SrFe${}_{2-x}$Ni${}_x$As${}_2$ denoted with an “a” are annealed (see text and footnote 43). All $T_c$ values follow a universal rate of suppression with $\Gamma$ well described by an Abrikosov-Gor'kov fit (dashed line) that is much weaker than expected for a superconductor with $s_{\pm }$ symmetry and interband scattering (dotted line) (Refs. 20, 30, and 32).