Superconductivity and structural variation of the electron-correlated layer systems Sr(Pd1-xTx)(2)Ge-2 (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)(2)Ge-2 layer system [Pd(4d(8)), T = Co(3d(7)), Ni(3d(8)), or Rh(4d(7)); 0 <= x <= 1] are reported. For the BaFe2As2-type tetragonal structure, the degenerate nd(7) or nd(8) orbitals of transitionmetal T are split by c-axis squeezed TGe4 tetrahedral crystal field in the T-Ge layer. For the isoelectronic Sr(Pd1-xNix)(2)Ge-2 system, the superconducting transition temperature T-c 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 nd(xz,yz) bands. For the Sr(Pd1-xRhx)(2)Ge-2 system, T-c decreases to 2.40 K with 25% of 4d(7) Rh substitution. For the Sr(Pd1-xCox)(2)Ge-2 system, T-c decreases sharply to 2.58 K with only 3% of 3d(7) Co substitution. No superconductivity is e! xpected for SrRh2Ge2 and SrCo2Ge2 with lower density of states in d(xz,yz) bands due to down shift of Fermi energy E-F by one less electron per transition metal. The lower T-c of the present electron-overdoped (nd(7) or nd(8)) compound is due to dispersive 3D-like nd(xz,yz) conduction bands with weak electron correlation, in comparison with 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 where electron contribution is from less dispersive 2D-like lower-lying 3d(xy) conduction band with stronger electron correlation.