Suppression of time reversal symmetry breaking superconcutivity in Pr(Os,Ru)$_{4}$Sb$_{12}$ and (Pr,La)Os$_{4}$Sb$_{12}$*

Zero-field muon spin relaxation ($\mu$SR) experiments have been carried out in the Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ and Pr$_{1-y}$La$_y$Os$_4$Sb$_{12}$ alloy systems to investigate broken time-reversal symmetry (TRS) in the superconducting state, signaled by the onset of a spontaneous static local magnetic field $B_{\rm s}$. In both alloy series $B_{\rm s}$ initially decreases linearly with solute concentration. Ru doping is considerably more efficient than La doping ,with a $\sim$$50\%$ faster initial decrease. The data suggest that broken TRS is suppressed for Ru concentration larger than 0.6, but persists for essentially all La concentrations. Our data support the theory of TRS-breaking superconductivity via crystal-field excitonic Cooper pairing of Koga, Matsumoto, and Shiba.