Muon spin relaxation in the heavy fermion system UPt3

We report muon spin rotation/relaxation (μSR) measurements of the heavy fermion superconductor UPt3. We observe the appearance of a spontaneous internal magnetic field at T = 475 mK, which is about 80 mK below Tc. This transition likely corresponds to the lower superconducting transition observed in specific-heat and ultrasound measurements. Our results provide microscopic evidence that the low-temperature/low-field phase in the superconducting phase diagram of UPt3 may break time-reversal symmetry.