Superconductivity in Novel Ge-Based Skutterudites: $\{\mathrm{Sr},\mathrm{Ba}\}{\mathrm{Pt}}_4{\mathrm{Ge}}_{12}$

Combining experiments and ab initio models we report on ${\mathrm{SrPt}}_4{\mathrm{Ge}}_{12}$ and ${\mathrm{BaPt}}_4{\mathrm{Ge}}_{12}$ as members of a novel class of superconducting skutterudites, where Sr or Ba atoms stabilize a framework entirely formed by Ge atoms. Below $T_c=5.35$ and 5.10 K for ${\mathrm{BaPt}}_4{\mathrm{Ge}}_{12}$ and ${\mathrm{SrPt}}_4{\mathrm{Ge}}_{12}$, respectively, electron-phonon coupled superconductivity emerges, ascribed to intrinsic features of the Pt-Ge framework, where Ge-$p$ states dominate the electronic structure at the Fermi energy.

Figure 1

Temperature dependent electrical resistivity $\rho$ of ${\mathrm{BaPt}}_4{\mathrm{Ge}}_{12}$ (a) and ${\mathrm{SrPt}}_4{\mathrm{Ge}}_{12}$ (b). Both insets show the field dependence of $\rho (T)$.

Figure 2

(a) Temperature dependent specific heat $C_p$ of ${\mathrm{SrPt}}_4{\mathrm{Ge}}_{12}$ and ${\mathrm{BaPt}}_4{\mathrm{Ge}}_{12}$ plotted as $C_p/T$ vs $T$ for 0 and 3 T. (b) Semilogarithmic plot $C_{\mathrm{es}}/\gamma T_c$ vs $T_c/T$. The inset shows the low temperature and low field susceptibility $M/H$ for both compounds.

Figure 3

Temperature dependent upper critical field ${\mu }_0{H}_{c2}$ of ${\mathrm{BaPt}}_4{\mathrm{Ge}}_{12}$ and ${\mathrm{SrPt}}_4{\mathrm{Ge}}_{12}$ as obtained from resistivity, magnetization, and specific heat measurements. The dashed and the dashed-dotted lines represent the WHH model.

Figure 4

DOS for ${\mathrm{BaPt}}_4{\mathrm{Ge}}_{12}$ (a), (b) and ${\mathrm{SrPt}}_4{\mathrm{Ge}}_{12}$ (c), (d) as derived from DFT calculations. Fermi energy is situated at zero energy. The upper half (a), (c) shows the density of states for a fully relativistic calculation, which includes spin-orbit coupling. The lower half (b), (d) of the figure presents the DOS of a standard nonrelativistic calculation. The total DOS is decomposed into the local DOS representative for one Ba(Sr), four Pt, and twelve Ge atoms. The relativistic effect is small but important, because it shifts the Fermi level of the nonrelativistic DOS almost into the maximum of the relativistic DOS.