The quasi-two-dimensional metal Sr₂RuO₄ is one of the best characterized unconventional superconductors, yet the nature of its superconducting order parameter is still under debate^1,2,3. This information is crucial to determine the pairing mechanism of Cooper pairs. Here we use ultrasound velocity to probe the superconducting state of Sr₂RuO₄. This thermodynamic probe is sensitive to the symmetry of the material, and therefore, it can help in identifying the symmetry of the superconducting order parameter^4,5. Indeed, we observe a sharp jump in the shear elastic constant c₆₆ as the temperature is increased across the superconducting transition. This directly implies that the superconducting order parameter is of a two-component nature. On the basis of symmetry arguments and given the other known properties of Sr₂RuO₄ (refs. ^6,7,8), we discuss which states are compatible with this requirement and propose that the two-component order parameter {d_xz; d_yz} is the most likely candidate.

准二维金属Sr ₂ RuO ₄是最典型的非常规超导体之一，但其超导阶数参数的性质仍在讨论^1,2,3中。此信息对于确定Cooper对的配对机制至关重要。在这里，我们使用超声速度来探测Sr ₂ RuO ₄的超导状态。该热力学探针对材料的对称性敏感，因此可以帮助识别超导阶数参数^4,5的对称性。确实，我们观察到了剪切弹性常数c ₆₆的急剧变化随着超导转变温度的升高。这直接暗示超导阶数参数具有两部分性质。根据对称性参数并给出Sr ₂ RuO ₄的其他已知性质（参考文献^6,7,8），我们讨论了哪些状态与该要求兼容，并提出了两组分阶数参数{ d _{x z} ; d _{y z} }是最有可能的候选者。