Ru NMR probe of spin susceptibility in the superconducting state ofSr2RuO4

We report ${}^{99}\mathrm{Ru}$ Knight-shift ${(}^{99}K)$ measurement on the high-quality ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ with a superconducting transition temperature $T_c\sim 1.5$ K. Its spin component, ${}^{99}{K}^s$ due to the core polarization effect caused by $\mathrm{R}\mathrm{u}-4\mathrm{}d$ spins, is nearly one order of magnitude larger than that of the ${}^{17}\mathrm{O}$ Knight shift ${(}^{17}{K}^s).\mathrm{}$ The $4d-\mathrm{s}\mathrm{p}\mathrm{i}\mathrm{n}$ susceptibility that has been more precisely measured from ${}^{99}{K}^s$ than ${}^{17}{K}^s$ shows no change across $T_c$ at all. The present result reinforces that ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ is the spin-triplet $p-\mathrm{w}\mathrm{a}\mathrm{v}\mathrm{e}$ superconductor in which electrons are bound together in parallel-spin pairs parallel to the ${\mathrm{RuO}}_2$ plane.