Spin-triplet superconductivity in Sr₂RuO₄ identified by ¹⁷O knight shift

Superconductivity - one of the best understood many-body problems in physics - has again become a challenge following the discovery of unconventional superconducting materials: these include heavy-fermion, organic and the high-transition-temperature copper oxide superconductors. In conventional superconductors, the electrons form superconducting Cooper pairs in a spin-singlet state, which has zero total spin (S = 0). In principle, Cooper pairs can also form in a spin-triplet state (S = 1), analogous to the spin-triplet 'p-wave' state of paired neutral fermions in superfluid ³He (ref. 4). At present, the heavy-fermion compound UPt₃ is the only known spin-triplet superconductor, although the layered oxide superconductor Sr₂RuO₄ (ref. 7) is believed, on theoretical grounds, to be a promising candidate. The most direct means of identifying the spin state of Cooper pairs is from measurements of their spin susceptibility, which can be determined by the Knight shift (as probed by nuclear magnetic resonance (NMR). Here we report Knight-shift measurements of Sr₂RuO₂ using ¹⁷O NMR. Our results show no change in spin susceptibility on passing through the superconducting transition temperature, which provides the definitive identification of Sr₂RuO₄ as a spin-triplet superconductor.