Superconducting order parameter in UTe$_2$ determined by Knight shift measurement

TL;DR

This study uses Knight shift measurements to show that UTe$_2$ is a spin-triplet superconductor with nearly unchanged spin susceptibility below T_c, indicating spin degrees of freedom in its superconducting state.

Contribution

It provides direct experimental evidence that UTe$_2$ exhibits spin-triplet pairing with a dominant B_{3u} state, advancing understanding of its unconventional superconductivity.

Findings

Knight shift remains unchanged below T_c for H along a axis

UTe$_2$ is confirmed as a spin-triplet superconductor

The dominant pairing state is B_{3u} symmetry

Abstract

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