Superconductivity is known as a phenomenon where electrical resistance becomes zero when a substance is cooled to a low temperature. In superconductivity, two electrons form a paired state (superconducting pair), and in most cases the spins of the two electrons are antiparallel (called the spin singlet state). Therefore, superconductivity is destroyed under a strong magnetic field. However, the uranium compound UTe 2 , which was recently reported to be superconducting, has a very strong superconducting state with respect to a magnetic field, and surprisingly 15 b-axis (hard magnetization axis: a direction in which magnetization is hard to be oriented) It exhibits the behavior that the transition temperature rises when a magnetic field of more than Tesla is applied [Fig. 1 (a)]. From this result, it is expected that in UTe 2 , superconductivity of spin triplet state in which the two electron spins in the pair state are aligned (parallel direction) is realized. In the spin triplet state, spin degrees of freedom remain even in the superconducting state, and it is expected that superconductivity will survive up to a high magnetic field if the spin takes the same direction as the external magnetic field. For its demonstration, it was necessary to directly observe the spin state in the superconducting state, but spin triplet state superconductivity is extremely rare, so detailed studies have not been conducted.
Here, we perform nuclear magnetic resonance (NMR) measurements on UTe 2 superconductors, which are microscopic measurements at the nuclear level, and UTe 2 is an unconventional superconductivity different from that normally found in metals. It is shown that the decrease in the spin susceptibility in the superconducting state is much smaller than that expected in the spin singlet state, and this material has a spin triplet state in which the spin degree of freedom survives in the superconducting state. I reported what I could think. [Fig. 1 (b)] We also reported data suggesting a gap between two superconducting materials by measuring the nuclear spin-lattice relaxation rate 1 / T 1 T. [Figure 2]
This research is a joint research with JAEA Advanced Research Center and Tohoku University Institute for Metal Research. The result was published as a letter in the Journal of the Physical Society of Japan and was selected as an Editors’ choice. Since it is OPEN SELECT, anyone can read it for free.
Fig. 1: (a) Superconducting phase diagram when a magnetic field is applied to the b-axis of UTe 2 crystals. (b) Spin magnetic susceptibility of UTe 2 when a magnetic field is applied to the b axis . The red double-headed arrow shows the expected reduction in spin singlet superconductivity estimated from the specific heat. The inset shows the vertical axis enlarged.
Fig. 2: Calculation results assuming temperature dependence of 1 / T 1 T and existence of two superconducting gaps
