Time-reversal symmetry in the hidden order and superconducting states of URu₂Si₂

The presence of a time-reversal symmetry-breaking field in the hidden order and superconducting states of URu₂Si₂ has been studied by muon spin relaxation (μSR) spectroscopy. The development of an internal magnetic field has been observed in the hidden order state. Longitudinal-field μSR experiments have shown that the internal magnetic field is static on the time scale of μSR. The presence of both a- and c-axis components of the internal magnetic field at the muon sites was revealed by transverse- field μSR experiments. This is in contrast to the pressure-induced antiferromagnetic state, where the internal magnetic field is parallel to the c-axis. We also found that the muon spin relaxation rate is further enhanced below the superconducting phase transition temperature, indicating further development of the internal magnetic field.