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 Tc in H ∥ b, and a slight decrease in the Knight shift along the b and c axes (Kb and Kc, respectively) below Tc at a low magnetic field H. Although the decrease in Kc vanished above 5.5 T, the decrease in Kb was independent of H up to 6.5 T. To clarify the origin of the shoulder signal and the trace of the decrease in Kb, 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 χAC measurements, which can survive up to 14.5 T (maximum H in the present measurement). Moreover, the decrease in Kb 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 Tc 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.
ASJC Scopus subject areas
- Physics and Astronomy(all)