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Superconductivity is a coherent quantum-mechanical state formed by electron pairs. Superconducting order parameters can be described using combinations of spin and orbital states. Although there are no spin degrees of freedom in the spin-singlet superconducting state, unique properties such as multiple superconducting states and spin rotation of a superconducting pair are expected in the spin-triplet superconducting state, owing to the presence of spin degrees of freedom. However, almost all discovered superconductors belong to the spin-singlet superconductors, with only a few spin-triplet superconductors reported thus far.

In 2018, superconductivity in UTe₂ was discovered. UTe₂ was initially classified as a spin-triplet superconductor owing to similarities with U-based ferromagnetic superconductors with spin-triplet pairing. In fact, characteristic features of the spin-triplet superconductor, such as an extremely large upper critical field H_c2 and superconducting multiple phases, were observed in UTe₂. In NMR measurements, our group obtained various results confirming the spin-triplet state.

In 2022, a significant improvement in sample quality was achieved by changing the synthesis method from chemical-vapor-transport to molten-salt flux method. The new samples showed an increase in T_c from 1.6 to 2.1 K. The residual specific heat coefficient at T → 0, which was approximately half of the normal-state value in the 1.6 K sample, was nearly zero in the 2.1 K sample, indicating the ultra-pure nature of the 2.1 K sample.

To elucidate the intrinsic superconducting symmetry of UTe₂, we performed NMR measurements on the 2.1 K sample. Because of the Meissner effect, bulk magnetization measurements cannot be used to determine spin susceptibility in the superconducting state; hence, NMR measurements are the only technique used. The results of NMR measurements for magnetic fields along three crystal axes revealed that the spin susceptibility in the a axis, which did not decrease in the 1.6 K samples, decreased significantly in the 2.1 K samples. However, the spin susceptibility in the b and caxes remained approximately the same as that in the 1.6 K samples. The results in the 2.1 K sample suggest that the possible superconducting state is the spin-triplet A_u state, which is the same pairing state as the superfluidity ³He B-phase. This superconducting state is a strong candidate for the topological superconductor, in which the Majorana surface state is anticipated.

Our findings highlight the importance of ultra-pure samples for investigating the essential behavior of superconductivity.

(written by H. Matsumura on behalf of all authors.)