Variation of superconductivity in the pseudoternary layer systems Sr(T _1-xT'_x)₂Ge₂ (T, T' = transition metals)

Variation of superconducting transition temperature Tc for the pseudoternary Sr(T1-xT'x)2Ge2 layer systems (T, T' Co, Ni, Rh, Pd; 0 ≤ x ≤ 1) are reported. For the system Sr(Pd1-xRhx)2Ge2, Tc decreases from 3.12 K for x 0, to 2.92 K for x 0.1, to 2.50 K for x 0.2, and extrapolated to 0 K for x 0.65. No Tc is expected for SrRh2Ge2. For the system Sr(Pd1-xCox)2Ge2, Tc decreases sharply to 2.80 K for x 0.01, 2.58 K for x 0.03, and extrapolates to 0 K for x 0.15. No Tc is also expected for SrCo2Ge2. In these systems, stronger Tc suppression for Co(3d⁷) and Rh(4d⁷) were due to lower density of states in dxz,yz bands with one less conduction electron per transition metal. Weaker Tc suppression for isoelectronic Ni is due to shift from Pd-4d⁸ to Ni-3d⁸ band. Tc decreases monotonically from 3.12 K for SrPd2Ge2 to 0.92 K for SrNi2Ge2. The lower Tc of the present electron-overdoped (nd⁷ or nd⁸) compounds are due to dispersive 3D-like ndxz,yz conduction bands with weaker electron correlation, as compared to the less-electron-doped (3d^6.1) 22 K superconductor BaFe1.8Co0.2As2 or the hole-doped (3d^5.9) 38 K superconductor Ba0.6K0.4Fe2As2 with additional less dispersive 2D-like 3dxy band contribution and stronger electron correlation.