Sr2RuO4 has stood as the leading candidate for a spin-triplet superconductor for 26 years. Recent NMR experiments have cast doubt on this candidacy, however, and it is difficult to find a theory of superconductivity that is consistent with all experiments. What is needed are symmetry-based experiments 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, requiring that the superconducting order parameter is two-component. A two-component p-wave order parameter, such as px+ipy, naturally satisfies this requirement. As this order parameter appears to be precluded by recent NMR experiments, we suggest that two other two-component order parameters, namely {dxz,dyz} or {dx2−y2,gxy(x2−y2)}, are now the prime candidates for the order parameter of Sr2RuO4.
@article{arxiv.2002.06130,
title = {Thermodynamic Evidence for a Two-Component Superconducting Order Parameter in Sr$_2$RuO$_4$},
author = {Sayak Ghosh and Arkady Shekhter and F. Jerzembeck and N. Kikugawa and Dmitry A. Sokolov and Manuel Brando and A. P. Mackenzie and Clifford W. Hicks and B. J. Ramshaw},
journal= {arXiv preprint arXiv:2002.06130},
year = {2023}
}