Recent observations [A.~Pustogow et al. Nature 574, 72 (2019)] of a drop of the $^{ 17}$O nuclear magnetic resonance (NMR) Knight shift in the superconducting state of Sr$_2$RuO$_4$ challenged the popular picture of a chiral odd-parity paired state in this compound. Here we use polarized neutron scattering to show that there is a $34 \pm 6$ % drop in the magnetic susceptibility at the ruthenium site below the superconducting transition temperature. Measurements are made at lower fields $H \sim \tfrac{ 1}{ 3} H_{ c2}$ than a previous study allowing the suppression to be observed. Our results are consistent with the recent NMR observations and rule out the chiral odd-parity $\mathbf{ d}=\hat{ \mathbf{ z}}(k_x\pm ik_y)$ state. The observed susceptibility is consistent with several recent proposals including even-parity $B_{ 1g}$ and odd-parity helical states.
Categories: cond-mat.supr-con cond-mat.str-el