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 c₆₆, 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.