This study investigates the spin susceptibility in U-based superconductor UTe2 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 UTe2. Although a tiny anomaly ascribed to the SC diamagnetic effect was observed just below the SC transition temperature Tc, 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 Tc. Considering the previous Knight shift results for H ∥ b and H ∥ c, the dominant SC state is determined to be B3u in the spin-triplet pairing, which is consistent with the spin anisotropy in the normal state. The present result shows that UTe2 is a spin-triplet superconductor with spin degrees of freedom.
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