Ultrasound evidence for a two-component superconducting order parameter in Sr 2 RuO 4

The quasi-two-dimensional metal Sr 2 RuO 4 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 Sr 2 RuO 4 . 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 c 66 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 Sr 2 RuO 4 (refs. 6 – 8 ), we discuss which states are compatible with this requirement and propose that the two-component order parameter { d x z ; d y z } is the most likely candidate.