Event Details

**Sr_2RuO_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. We use resonant ultrasound spectroscopy — a unique method for measuring all elastic moduli in a single experiment — to measure the entire symmetry-resolved elastic tensor of **Sr_2RuO_4** through the superconducting transition. We have discovered a thermodynamic discontinuity in the shear elastic modulus c66, which requires that the superconducting order parameter has two components. The standard interpretation of such a result would be the two-component p-wave order parameter **px + ipy**. However, this order parameter appears to be precluded by recent NMR experiments, and we therefore suggest two other rather exotic two-component order parameters - **d_xz + i d_yz**, or **d_x^2-y^2 + i g _xz(x^2-y^2)**. I will discuss the implications of this result for the long-standing problem of superconductivity in **Sr_2RuO_4**, as well as the recently discovered superconductor **UTe_2**.