Variation of superconductivity in the pseudoternary layer systems Sr(T1−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 (3d6.1) 22 K superconductor BaFe1.8Co0.2As2 or the hole-doped (3d5.9) 38 K superconductor Ba0.6K0.4Fe2As2 with additional less dispersive 2D-like 3dxy band contribution and stronger electron correlation.