No Detectable Change in In-Plane 29Si Knight Shift in the Superconducting State of URu2Si2

Using a high-quality single crystal of URu2Si2, we performed Si-29 Knight shift measurements with an applied magnetic field H-a along the crystalline a-axis, i.e., along the magnetic hard axis, for temperatures down to 50 mK. The narrow, single-peak Si-29 NMR spectrum obtained from this crystal allowed us to examine the temperature variation of the Knight shift K(T), and hence, that of the variation of the spin susceptibility with sensitivity on the order of 0.001%. At this precision, no observable change in K(T) was detected from 50 mK up to the superconducting transition at T-SC(H-a) similar to 1K. Our results correspond to either an extremely small spin susceptibility in the hidden-order state along the magnetic hard axis or to possible spin-triplet superconductivity in this system.