Superconducting Order Parameter in UTe₂ Determined by Knight Shift Measurement

Abstract

This study investigates the spin susceptibility in U-based superconductor\nUTe$_2$ in the superconducting (SC) state by using Knight shift measurements\nfor a magnetic field $H$ along the $a$ axis, which is the magnetic easy axis of\nUTe$_2$. Although a tiny anomaly ascribed to the SC diamagnetic effect was\nobserved just below the SC transition temperature $T_{\\rm c}$, the $a$-axis\nKnight shift in the SC state shows no significant decrease, following the\nextrapolation from the normal-state temperature dependence. This indicates that\nthe spin susceptibility is nearly unchanged below $T_{\\rm c}$. Considering the\nprevious Knight shift results for $H \\parallel b$ and $H \\parallel c$, the\ndominant SC state is determined to be $B_{\\rm 3u}$ in the spin-triplet pairing,\nwhich is consistent with the spin anisotropy in the normal state. The present\nresult shows that UTe$_2$ is a spin-triplet superconductor with spin degrees of\nfreedom.\n