  Superconducting (SC) state has spin and orbital degrees of freedom, and
spin-triplet superconductivity shows multiple SC phases due to the presence of
these degrees of freedom. However, the observation of spin-direction rotation
occurring inside the SC state (SC spin rotation) has hardly been reported.
UTe2, a recently discovered topological superconductor, exhibits various SC
phases under pressure: SC state at ambient pressure (SC1), high-temperature SC
state above 0.5 GPa (SC2), and low-temperature SC state above 0.5 GPa (SC3). We
performed nuclear magnetic resonance and AC susceptibility measurements on
single-crystal UTe2. The b-axis spin susceptibility remains unchanged in SC2,
unlike in SC1, and decreases below the SC2-SC3 transition with spin modulation.
These unique properties in SC3 arise from the coexistence of two SC order
parameters. Our NMR results confirm the spin-triplet superconductivity with SC
spin parallel to b in SC2, and unveil the remaining of spin degrees of freedom
in superconducting UTe2.
