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

Sr 2 RuO 4 has stood as the leading candidate for a spin-triplet superconductor for 26 years 1 . However, recent NMR experiments have cast doubt on this candidacy 2 , 3 and it is difficult to find a theory of superconductivity that is consistent with all experiments. The order parameter symmetry for this material therefore remains an open question. Symmetry-based experiments are needed 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 , which implies that the superconducting order parameter has two components. A two-component p -wave order parameter, such as p x + i p y , naturally satisfies this requirement. As this order parameter appears to have been precluded by recent NMR experiments, we suggest that two other two-component order parameters, namely { d x z , d y z } and { d x 2 − y 2 , g x y ( x 2 − y 2 ) } , are now the prime candidates for the order parameter of Sr 2 RuO 4 . Ultrasound measurements show that the superconducting order parameter in strontium ruthenate must have two components.