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The physical picture of unconventional superconductivity (SC) in Sr₂RuO₄, which was considered a solid-state analogue to the superfluid ³He-A phase for more than two decades [1,2], has been recently overturned [3]. Here we use ¹⁷O NMR spectroscopy to probe the nature of the SC state in Sr₂RuO₄ and its evolution with magnetic field. The drop of Knight shift K is measured in the limit T → 0 across the field-tuned transition down to B/B_c2 < 0.2. While K includes contributions of both field-induced quasiparticles (QP) and a possible spin polarization of the condensate (of order unity for a p-wave state), the specific heat C/T includes only the QP term. By comparing the field dependence of K and C/T, we establish an upper bound for the condensate magnetic response of < 10% of the normal-state susceptibility, which is sufficient to exclude odd-parity candidates [4].

[1] J. Phys. Condens. Matter 7, L643 (1995)
[2] Nature 396, 658–660 (1998)
[3] Nature 574, 72–75 (2019)
[4] arXiv:2007.13730