Reduction of the Spin Susceptibility in the Superconducting State of Sr2 RuO4 Observed by Polarized Neutron Scattering

Recent observations [A. Pustogow et al., Nature (London) 574, 72 (2019)., 10.1038/s41586-019-1596-2] of a drop of the O 17 nuclear magnetic resonance (NMR) Knight shift in the superconducting state of Sr₂ RuO₄ challenged the popular picture of a chiral odd-parity paired state in this compound. Here we use polarized neutron scattering (PNS) to show that there is a 34 ±6 % drop in the magnetic susceptibility at the Ru site below the superconducting transition temperature. We measure at lower fields H ∼1/3 H_{c 2} than a previous PNS study allowing the suppression to be observed. The PNS measurements show a smaller susceptibility suppression than NMR measurements performed at similar field and temperature. Our results rule out the chiral odd-parity d =z ^(k_x±i k_y) state and are consistent with several recent proposals for the order parameter including even-parity B_{1 g} and odd-parity helical states.