Superconductivity and structural variation of the electron-correlated layer systems Sr(Pd1−xTx)2Ge2 (T = Co, Ni, Rh; 0⩽x⩽1)

Abstract

Superconductivity variations deduced from the x-ray diffraction and the magnetic and heat-capacity measurements in the pseudoternary Sr(Pd1−xTx)2Ge2 layer system [Pd(4d8), T = Co(3d7), Ni(3d8), or Rh(4d7); 0⩽x⩽1] are reported. For the BaFe2As2-type tetragonal structure, the degenerate nd7 or nd8 orbitals of transition metal T are split by c-axis squeezed TGe4 tetrahedral crystal field in the T-Ge layer. For the isoelectronic Sr(Pd1−xNix)2Ge2 system, the superconducting transition temperature Tc decreases monotonically from 3.12 K for 4d-band SrPd2Ge2 to 0.92 K for 3d-band SrNi2Ge2, where major contributions of conduction electrons are from the half filled dispersive three-dimensional (3D)-like upper-lying ndxz,yz bands. For the Sr(Pd1−xRhx)2Ge2 system, Tc decreases to 2.40 K with 25% of 4d7 Rh substitution. For the Sr(Pd1−xCox)2Ge2 system, Tc decreases sharply to 2.58 K with only 3% of 3d7 Co substitution. No superconductivity is expected for SrRh2Ge2 and SrCo2Ge2 with lower density of states in dxz,yz bands due to down shift of Fermi energy EF by one less electron per transition metal. The lower Tc of the present electron-overdoped (nd7 or nd8) compound is due to dispersive 3D-like ndxz,yz conduction bands with weak electron correlation, in comparison with the less-electron-doped (3d6.1) 22-K superconductor BaFe1.8Co0.2As2 or the hole-doped (3d5.9) 38-K superconductor Ba0.6K0.4Fe2As2 where electron contribution is from less dispersive 2D-like lower-lying 3dxy conduction band with stronger electron correlation.

Source-reported materials — not catalogue approval

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