Superconductivity with T _c ≈ 7 K under pressure for Cu- and Au-doped BaFe₂As₂

It is noteworthy that chemical substitution of BaFe₂As₂ (122) with the noble elements Cu and Au gives superconductivity with a maximum T _c ≈ 3 K, while Ag substitution (Ag-122) stays antiferromagnetic. For Ba(Fe_1-x TM _x )₂As₂, TM = Cu, Au, or Ag, and by doping an amount of x = 0.04, a-lattice parameter slightly increases (0.4%) for all TM dopants, while c-lattice decreases (-0.2%) for TM = Cu, barely moves (0.05%) for Au, and increases (0.2%) for Ag. Despite the naive expectation that the noble elements of group 11 should affect the quantum properties of 122 similarly, they produce significant differences extending to the character of the ground state. For the Ag-122 crystal, evidence of only a filamentary superconductivity is noted with pressure. However, for Au and Cu doping (x ≈ 0.03) we find a substantial improvement in the superconductivity, with T _c increasing to 7 K and 7.5 K, respectively, under 20 kbar of pressure. As with the ambient pressure results, the identity of the dopant therefore has a substantial impact on the ground state properties. Density functional theory calculations corroborate these results and find evidence of strong electronic scattering for Au and Ag dopants, while Cu is comparatively less disruptive to the 122 electronic structure.