Superconductivity variations deduced from the x-ray diffraction and the magnetic and heat-capacity measurements in the pseudoternary Sr(Pd 1-xT x) 2Ge 2 layer system [Pd(4d8), T= Co(3d7), Ni(3d8), or Rh(4d7); 0≤x≤1] are reported. For the BaFe 2As 2-type tetragonal structure, the degenerate nd7 or nd8 orbitals of transition metal T are split by c-axis squeezed TGe 4 tetrahedral crystal field in the T-Ge layer. For the isoelectronic Sr(Pd 1-xNi x) 2Ge 2 system, the superconducting transition temperature T c decreases monotonically from 3.12 K for 4d-band SrPd 2Ge 2 to 0.92 K for 3d-band SrNi 2Ge 2, 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(Pd 1-xRh x) 2Ge 2 system, T c decreases to 2.40K with 25% of 4d7 Rh substitution. For the Sr(Pd 1-xCo x) 2Ge 2 system, T c decreases sharply to 2.58 K with only 3% of 3d7 Co substitution. No superconductivity is expected for SrRh 2Ge 2 and SrCo 2Ge 2 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 (nd7 or nd8) compound is due to dispersive 3D-like nd xz,yz conduction bands with weak electron correlation, in comparison with the less-electron-doped (3d6.1) 22-K superconductor BaFe 1.8Co 0.2As 2 or the hole-doped (3d5.9) 38-K superconductor Ba 0.6K 0.4Fe 2As 2 where electron contribution is from less dispersive 2D-like lower-lying 3d xy conduction band with stronger electron correlation.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 2012 Jan 31|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics