  Zero and longitudinal field muon spin rotation (muSR) experiments were
performed on the superconductors PrPt4Ge12 and LaPt4Ge12. In PrPt4Ge12 below Tc
a spontaneous magnetization with a temperature variation resembling that of the
superfluid density appears. This observation implies time-reversal symmetry
(TRS) breaking in PrPt4Ge12 below Tc = 7.9 K. This remarkably high Tc for an
anomalous superconductor and the weak and gradual change of Tc and of the
related specific heat anomaly upon La substitution in La_(1-x)Pr_xPt_4Ge_(12)
suggests that the TRS breaking is due to orbital degrees of freedom of the
Cooper pairs.
