Thermodynamic evidence for a two-component superconducting order parameter in Sr2RuO4

Sr2RuO4 has stood as the leading candidate for a spin-triplet superconductor for 26 years1. However, recent NMR experiments have cast doubt on this candidacy(2,3) and it is difficult to find a theory of superconductivity that is consistent with all experiments. The order parameter symmetry for this material therefore remains an open question. Symmetry-based experiments are needed that can rule out broad classes of possible superconducting order parameters. Here, we use resonant ultrasound spectroscopy to measure the entire symmetry-resolved elastic tensor of Sr2RuO4 through the superconducting transition. We observe a thermodynamic discontinuity in the shear elastic modulus c(66), which implies that the superconducting order parameter has two components. A two-component p-wave order parameter, such as p(x) + ip(y), naturally satisfies this requirement. As this order parameter appears to have been precluded by recent NMR experiments, we suggest that two other two-component order parameters, namely {d(xz); d(yz)} and {d(x2-y2); g(xy(x2 -y2))},, are now the prime candidates for the order parameter of Sr2RuO4.

Automated summary

Sr2RuO4 has been considered the leading candidate for a spin-triplet superconductor for 26 years, but recent experiments have raised doubts. Observations of a thermodynamic discontinuity in the shear elastic modulus suggest a two-component superconducting order parameter. Therefore, two other two-component order parameters are now seen as the prime candidates for the order parameter of Sr2RuO4.