NMR investigation on the high-field superconducting phases of UTe₂ in fields along the hard magnetic axis

The unconventional superconductor UTe₂ is known to exhibit a sequence of field-induced phases in the presence of magnetic field (B_b) aligned with the hard magnetic axis (b-axis). At relatively low field (B_b < 14 T), its properties are interpreted as consistent with a triplet-paired superconducting (SC) ground state (dubbed low-field SC or LFSC phase) [1]. At higher field a different high-field SC phase (HFSC) is stabilized, for which the symmetry is still debated. Recently, a molten-salt-flux technique has led to an improved single crystal quality, with the zero-field SC critical temperature reaching T_c = 2.1 K. These higher-quality crystals are reported to host an intermediate-field SC phase (IFSC) [2]; to date, the properties of the IFSC phase remain largely unexplored. Here, we report the results of ¹²⁵Te nuclear magnetic resonance (NMR) measurements, performed in the normal and SC states on a UTe₂ single crystal grown by the molten-salt-flux technique, in magnetic fields up to B_b = 30 T. Our observations suggest that the normal and SC state properties may greatly depend on the crystal quality and growth techniques.



[1] H. Matsumura et al, J. Phys. Soc. Jpn. 92, 063701 (2023).

[2] H. Sakai et al, Phys. Rev. Lett. 130, 196002 (2023).