Even parity superconducting state of the strongly correlated Fermi Liquid Sr₂RuO₄

Unambiguous identification of the superconducting order parameter symmetry of the strongly correlated Fermi Liquid Sr₂RuO₄ has remained elusive for more than a quarter century. While a chiral p-wave ground state analogue to superfluid ³He-A was ruled out only very recently [1], other proposed p-wave scenarios are still considered viable. We study the field-dependence of the ¹⁷O Knight shift at very low temperature, and by way of comparing to corresponding specific heat measurements, conclude that the shift results can be accounted for by the expected field-induced quasiparticle response only. This allows us to assign an upper bound for the condensate magnetic response of <10% measured relative to the normal state response [2]. The results are sufficient to exclude odd-parity candidates. Implications for these results will be discussed in the broader context of pertinent experimental findings and theoretical proposals.

[1] A. Pustogow, et al., Nature 574, 72–75 (2019).
[2] A. Chronister, et al., arXiv:2007.13730.