Evidence for spin singlet pairing with strong uniaxial anisotropy in URuSi using nuclear magnetic resonance

Hattori, Taisuke; Sakai, Hironori   ; Tokunaga, Yo  ; Kambe, Shinsaku  ; Matsuda, Tatsuma*; Haga, Yoshinori   

In order to identify the spin contribution to superconducting pairing compatible with the so-called "hidden order", Si nuclear magnetic resonance measurements have been performed using a high-quality single crystal of URuSi. A clear reduction of the Si Knight shift in the superconducting state has been observed under magnetic field applied along the crystalline axis, corresponding to the magnetic easy axis. These results provide direct evidence for the formation of spin-singlet Cooper pairs. Consequently, results indicating a very tiny change of in-plane Knight shift reported previously demonstrate extreme uniaxial anisotropy for the spin susceptibility in the hidden order state.