Fully gapped pairing state in spin-triplet superconductor UTe 2

The recently discovered superconductor UTe 2 is a promising candidate for spin-triplet superconductors, but the symmetry of the superconducting order parameter remains highly controversial. Here, we determine the superconducting gap structure by the thermal conductivity of ultraclean UTe 2 single crystals. We find that the a -axis thermal conductivity divided by temperature κ/ T in zero-temperature limit is vanishingly small for both magnetic field H ‖ a and H ‖ c axes up to H / H c 2 ∼ 0.2, demonstrating the absence of nodes around the a axis contrary to the previous belief. The present results, combined with the reduction of nuclear magnetic resonance Knight shift, indicate that the superconducting order parameter belongs to the isotropic A u representation with a fully gapped pairing state, analogous to the B phase of superfluid 3 He. These findings reveal that UTe 2 is likely to be a long-sought three-dimensional strong topological superconductor, hosting helical Majorana surface states on any crystal plane.