Superconductivity in the New Electron-correlated 122-layer System Ca$T_2$Ge$_2$ ($T$ = Ir, Pd, Pt)

Superconductivity were observed in the new 122-layer compounds CaIr$_2$Ge$_2$ ($T_c$ = 5.4 K) and CaPd$_2$Ge$_2$ ($T_c$ = 2.5 K) with the BaFe$_2$As$_2$-type body-centered-tetragonal structure (bct, space group $I4/mmm$). For the pseudoternary Ca(Ir$_{1-x}$Pt$_x$)$_2$Ge$_2$ system, superconducting transition $T_c$ decreases from 5.4 K for CaIr$_2$Ge$_2$, to 3.8 K for $x = 0.1$, 3.0 K for $x = 0.2$, 2.7 K for $x = 0.3$, 2.2 K for $x = 0.5$, and below 2 K for $x > 0.5$. In addition to the 122-bct phase, x-ray powder diffraction pattern shows the appearance of a non-superconducting 122-monoclinic phase (space group $P2_1$). No $T_c$ above 2 K was observed for the single-phase monoclinic compound CaPt$_2$Ge$_2$. Higher $T_c$ in the bct CaIr$_2$Ge$_2$ is due to a strong quasi-2D $5d_{xz,yz}$-$4p$-$5d_{xz,yz}$ hybridization in the Ir-Ge-Ir layer with the squeezed-along-$c$-axis IrGe$_4$ tetragonal crystal field and the Ir-$5d$ spin-orbital interaction. For the 11-orthorhombic precursor (Ir$_{1-x}$Pt$_x$)Ge (space group $Pnma$), $T_c$ decreases from 4.8 K for IrGe, to 3.6 K for $x = 0.1$, 2.3 K for $x = 0.2$, and below 2 K for $x \geq 0.3$, with a reported low $T_c$ of 0.4 K for PtGe.