Although the properties of most superconducting materials are well described by the theory of Bardeen, Cooper and Schrieffer (BGS), considerable effort has been devoted to the search for exotic superconducting systems in which BCS theory does not apply. The transition to the superconducting state in conventional BCS superconductors involves the breaking of gauge symmetry only, whereby the wavefunction describing the Cooper pairs—the paired electron states responsible for superconductivity—adopt a definite phase. In contrast, a signature of an unconventional superconducting state is the breaking of additional symmetries, which can lead to anisotropic pairing (such as the 'd-wave' symmetry observed in the copper oxide superconductors) and the presence of multiple superconducting phases (as seen in UPt_3 and analogous behaviour in superfluid ~3He; refs 3-5). Here we report muon spin-relaxation measurements on the superconductor Sr_2RuO_4 that reveal the spontaneous appearance of an internal magnetic field below the transition temperature: the appearance of such a field indicates that the superconducting state in this material is characterized by the breaking of time-reversal symmetry. These results, combined with other symmetry considerations, suggest that superconductivity in Sr_2RuO_4 is of 'p-wave' (odd-parity) type, analogous to superfluid ~3He.
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