Muon spin relaxation in UPt3

We report muon spin rotation-relaxation measurements of the heavy fermion superconductor UPt3. The broadening of the transverse field muon precession signal sets in approximately 60 mK below Tc, a temperature which corresponds to the lower superconducting transition. In zero applied magnetic field, we observe an increase in the internal magnetic field within the superconducting state which can be explained if the ‘‘lower superconducting phase’’ in the H-T phase diagram of UPt3 is characterized by broken time-reversal symmetry.