Suppression of time-reversal symmetry breaking superconductivity in Pr(Os1−xRux)4Sb12and Pr1−yLayOs4Sb12

Zero-field muon spin relaxation experiments have been carried out in the Pr(Os${}_{1\ensuremath{-}x}$Ru${}_{x}$)${}_{4}$Sb${}_{12}$ and Pr${}_{1\ensuremath{-}y}$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 ${B}_{s}$. In both alloy series ${B}_{s}$ initially decreases linearly with solute concentration. Ru doping is considerably more efficient than La doping, with a $~$50% faster initial decrease. The data suggest that broken TRS is suppressed for Ru concentration $x\ensuremath{\gtrsim}0.6$ but persists for essentially all La concentrations. Our data support a crystal-field excitonic Cooper pairing mechanism for TRS-breaking superconductivity.