Defects and broken time-reversal symmetry in superconducting {\boldmath Pr(Os,Ru)$_{4}$Sb$_{12}$}*

Muon spin relaxation studies of a spontaneous local field $H_\mu$, previously observed in the superconducting state of PrOs$_4$Sb$_{12}$ and attributed to broken time-reversal symmetry, have been extended to Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ alloys. In flux-grown single crystals $H_\mu$ is strongly suppressed but remains observable for $x \le 0.2$. In powder samples prepared by solid state reaction, however, no field is observed for $x = 0.1$ or 0.2. Muon spin relaxation due to dynamic $^{141}$Pr nuclear spin fluctuations is also reduced in the powders. Both results can both be understood if the density of microscopic defects is smaller in the powders: defects increase muon spin relaxation by $^{141}$Pr spins, and supercurrents associated with defects are theoretically predicted to create spontaneous fields in TRS-breaking superconductors. Our results are strong experimental evidence for this prediction.