For two decades, strontium ruthenate has been the leading candidate for a solid-state realization of chiral p-wave superconductivity. This is important because this form of superconductivity has an unusual triplet order parameter and is predicted to host Majorana fermions. Now, Andrej Pustogow and co-workers have ruled out this interpretation by conducting careful nuclear magnetic resonance experiments.
This technique is sensitive to the symmetry of the order parameter via the change in the Knight shift upon entering the superconducting state. The authors found a substantial reduction at the critical temperature, indicative of a decrease in the magnetization and therefore an absence of the spin polarization that would be expected for a triplet state. This demonstrates that the superconductivity in strontium ruthenate is likely of singlet character.
There are still many exotic options for what exactly that state might be. These results mark a new stage in our understanding of both triplet superconductivity and strontium ruthenate.
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Abergel, D. Triplet no more. Nat. Phys. 15, 1105 (2019). https://doi.org/10.1038/s41567-019-0715-1