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A. Pustogow, A. Chronister, S. Brown:
"Evidence for even parity unconventional superconductivity in Sr2RuO4";
Hauptvortrag: Journées de la Matière Condensée 2021, Rennes, France (eingeladen); 24.08.2021 - 27.08.2021.

The physical picture of unconventional superconductivity (SC) in Sr2RuO4 has remained controversial since its discovery. While the system was considered a solid-state analogue to the superfluid 3He-A phase for more than two decades, the broadly discussed chiral triplet scenario has been recently overturned. However, other proposed triplet-pairing states are still viable. Establishing the condensate magnetic susceptibility is crucial as it reveals a sharp distinction between even parity (singlet) and odd parity (triplet) pairing, since the superconducting condensate is magnetically polarizable only in the latter case.
Here we use 17O NMR spectroscopy to probe the nature of the SC state in Sr2RuO4 and its evolution with magnetic field. The Knight shift K, being sensitive to the spin polarization, is measured in the limit T --> 0 across the field-tuned transition down to B/Bc2 < 0.2. While K includes contributions of both the 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 their field dependences, we find that the drop of K in the SC state coincides with the reduction of C/T - leaving no room for a finite condensate contribution. This way, 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.