Fully gapped pairing state in spin-triplet superconductor UTe.

The recently discovered superconductor UTe 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 single crystals. We find that the -axis thermal conductivity divided by temperature κ/ in zero-temperature limit is vanishingly small for both magnetic field ‖ and ‖ axes up to / ∼ 0.2, demonstrating the absence of nodes around the 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 representation with a fully gapped pairing state, analogous to the B phase of superfluid He. These findings reveal that UTe is likely to be a long-sought three-dimensional strong topological superconductor, hosting helical Majorana surface states on any crystal plane.