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The quasi-2D metal Sr$_2$RuO$_4$ is one of the best characterizedunconventional superconductors, yet the nature of its superconducting orderparameter is still highly debated. This information is crucial to determine thepairing mechanism of Cooper pairs. Here we use ultrasound velocity to probe thesuperconducting state of Sr$_2$RuO$_4$. This thermodynamic probe issymmetry-sensitive and can help to identify the superconducting order symmetry.Indeed, we observe a sharp jump in the shear elastic constant $c_{66}$ as thetemperature is raised across the superconducting transition at $T_c$. Thisdirectly implies that the superconducting order parameter is of a two-componentnature. Based on symmetry argument and given the other known properties ofSr$_2$RuO$_4$, we discuss what states are compatible with this requirement andpropose that the two-component order parameter, namely $\lbrace d_{xz}; d_{yz}\rbrace$, is the most likely candidate.

Ultrasound evidence for a two-component superconducting order parameter in Sr$_2$RuO$_4$