Superconductivity of PrPt$_4$Ge$_{12}$ and LaPt$_4$Ge$_{12}$ filled skutterudite compounds

Among the recently discovered rare-earth filled skutterudites $R$Pt$_4$Ge$_{12}$ ($R$ = La--Nd, Sm, Eu) there are two superconducting compounds - LaPt$_4$Ge$_{12}$ and PrPt$_4$Ge$_{12}$ [1]. In the latter Pr is in a well-separated singlet crystal field ground state. This allows for a rather high $T_c$ of 7.9\,K - comparable to the $T_c$ = 8.3\,K of LaPt$_4$Ge$_{12}$. By muon spin rotation and specific heat measurements a superconducting gap with point nodes and time-reversal symmetry (TRS) breaking is found below $T_c$ for PrPt$_4$Ge$_{12}$ while the La compound does not show TRS breaking [2,3]. Interestingly, samples of Pr$_x$La$_{1-x}$Pt$_4$Ge$_{12}$ have very similar $T_c$. We discuss the implications of these observations for the superconducting order parameters of these compounds.\newline [1] R.Gumeniuk et al., Phys.Rev.Lett. 100, 017002 [2] A.Maisuradze et al., Phys.Rev.Lett. 103, 147002 [3] A.Maisuradze et al., submitted