Time-reversal symmetry breaking in superconducting Pr$_{1-x}$Ce$_{x}$Pt$_4$Ge$_{12}$*

Zero-field muon-spin-relaxation ($\mu$SR) experiments were performed on the superconductors Pr$_{1-x}$Ce$_{x}$Pt$_4$Ge$_{12}$ ($x < 0.2$). The results reveal the spontaneous appearance of static internal magnetic fields below the superconducting transition temperature ($T_c$). This observation implies time-reversal symmetry breaking in Pr$_{1-x}$Ce$_{x}$Pt$_4$Ge$_{12}$ ($x < 0.2$) below $T_c$.