Fully gapped pairing state in spin-triplet superconductor UTe₂

Spin-triplet superconductors provide an ideal platform for realizing topological superconductivity with emergent Majorana quasiparticles. The promising candidate is the recently discovered superconductor UTe₂. However, the symmetry of the superconducting order parameter remains highly controversial. Here we investigate the superconducting gap structure by the thermal conductivity of ultra-clean UTe₂ single crystals with an extremely high residual resistivity ratio over 200. In zero field, the residual thermal conductivity κ/T (T → 0) vanishes, indicating the absence of line nodes. In stark contrast to previous results of thermal conductivity and penetration depth, the thermal conductivity in magnetic fields H in zero temperature limit is vanishingly small for both H//a and H//c up to 60% of upper critical fields. These results provide evidence for the absence of the point nodes, i.e. fully gapped superconductivity. Provided that Fermi surface is present along the c axis direction, the superconducting order parameter most likely belongs to the A_u representation, reminiscent of the B phase of superfluid ³He. These findings suggest that UTe₂ is a long-sought three-dimensional (3D) strong topological superconductor characterized by a nontrivial 3D winding number, hosting helical Majorana surface states on any plane.