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

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^1–3. 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. ^6–8), 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.