Muon spin relaxation in heavy fermion systems

Heavy fermion systems have received a great deal of study by a wide variety of techniques, includingμSR. In a number of systems, coexisting superconducting and magnetic states have been reported, leading to speculation of an intimate connection between magnetism and superconductivity in these compounds. We observe a spontaneous magnetic field in the superconducting phase of UPt3. In addition, the broadening of the transverse field muon precession signal only onsets approximately 60 mK below the superconductingTc. Our results provide evidence that the “lower superconducting phase” in theH-T phase diagram of UPt3 is characterised by broken time-reversal symmetry. Measurements of URu2Si2 and CeCu2.2Si2 indicate that the magnetically ordered volume fraction is temperature dependent in both systems.