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.