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

• DOI: 10.1038/s41567-020-1032-4
• Publication Date: 2020-09-21T16:05:04Z
• Authors: Sayak Ghosh; Arkady Shekhter; F. Jerzembeck; N. Kikugawa; Dmitry A. Sokolov; Manuel Brando; A. P. Mackenzie; Clifford W. Hicks; B. J. Ramshaw
• Keywords: Component (thermodynamics); Resonant ultrasound spectroscopy

Sr$_2$RuO$_4$ 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 Sr$_2$RuO$_4$ through the superconducting transition. We observe a thermodynamic discontinuity in the shear elastic modulus $c_{66}$, requiring that the superconducting order parameter is two-component. A two-component $p$-wave order parameter, such as $p_x+i p_y$, 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 $\left\{d_{xz},d_{yz}\right\}$ or $\left\{d_{x^2-y^2},g_{xy(x^2-y^2)}\right\}$, are now the prime candidates for the order parameter of Sr$_2$RuO$_4$.