Inhomogeneous superconducting state probed by ¹²⁵Te NMR on UTe₂

UTe₂ is a recently discovered promising candidate for a spin-triplet superconductor. In contrast to conventional spin-singlet superconductivity, spin-triplet superconductivity possesses spin and angular momentum degrees of freedom. To detect these degrees of freedom and obtain the solid evidence of spin-triplet superconductivity in UTe₂, we have performed ¹²⁵Te-NMR measurements, which are sensitive to the local spin susceptibility at a nuclear site. We previously reported that the shoulder signal appears in NMR spectra below the superconducting (SC) transition temperature T_c in H ∥ b, and a slight decrease in the Knight shift along the b and c axes (K_b and K_c, respectively) below T_c at a low magnetic field H. Although the decrease in K_c vanished above 5.5 T, the decrease in K_b was independent of H up to 6.5 T. To clarify the origin of the shoulder signal and the trace of the decrease in K_b, we compared the ¹²⁵Te-NMR spectra obtained when H ∥ b and H ∥ c and measured the ¹²⁵Te-NMR spectra for H ∥ b up to 14.5 T. The intensity of the shoulder signal observed for H ∥ b has a maximum at ∼6 T and vanishes above 10 T, although the superconductivity is confirmed by the χ_AC measurements, which can survive up to 14.5 T (maximum H in the present measurement). Moreover, the decrease in K_b in the SC state starts to be small around 7 T and almost zero at 12.5 T. This indicates that the SC spin state gradually changes with the application of H. Meanwhile, in H ∥ c, an unexpected broadening without the shoulder signals was observed below T_c at 1 T, and this broadening was quickly suppressed with increasing H. We construct the H–T phase diagram for H ∥ b and H ∥ c based on the NMR measurements and discuss possible SC states with the theoretical consideration. We suggest that the inhomogeneous SC state characterized by the broadening of the NMR spectrum originates from the spin degrees of freedom.