Fully gapped pairing state in spin-triplet superconductor UTe ₂

Abstract

The recently discovered superconductor UTe₂ 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 UTe₂ single crystals. We find that the <i>a</i>-axis thermal conductivity divided by temperature κ/<i>T</i> in zero-temperature limit is vanishingly small for both magnetic field <b><i>H</i></b>‖<i>a</i> and <b><i>H</i></b>‖<i>c</i> axes up to <i>H</i>/<i>H</i>_<i>c</i>2 ∼ 0.2, demonstrating the absence of nodes around the <i>a</i> 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 <i>A_u</i> representation with a fully gapped pairing state, analogous to the B phase of superfluid ³He. These findings reveal that UTe₂ is likely to be a long-sought three-dimensional strong topological superconductor, hosting helical Majorana surface states on any crystal plane.