The complex structure of the Remeika phases, the intriguing quantum states they display, and their low carrier concentrations are strong motivations to study the nature of their superconducting phases. In this Letter, the microscopic properties of the superconducting phase of single-crystalline ${\mathrm{Lu}}_3{\mathrm{Os}}_4{\mathrm{Ge}}_{13}$ are investigated by muon-spin relaxation and rotation ($\mu \mathrm{SR}$) measurements. The zero-field $\mu \mathrm{SR}$ data reveal the presence of spontaneous static or quasistatic magnetic fields in the superconducting state, breaking time-reversal symmetry; the associated internal magnetic-field scale is found to be exceptionally large ($\simeq$ 0.11 mT). Furthermore, transverse-field $\mu \mathrm{SR}$ measurements in the vortex state of ${\mathrm{Lu}}_3{\mathrm{Os}}_4{\mathrm{Ge}}_{13}$ imply a complex gap function with significantly different strengths on different parts of the Fermi surface. Although our measurements do not completely determine the order parameter, they strongly indicate that electron-electron interactions are essential to stabilizing pairing in the system, thus, demonstrating its unconventional nature.

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• Skutterudites
• Unconventional superconductors
• T-symmetry
• Muon spin resonance