Fully gapped pairing state in spin-triplet superconductor UTe$_2$

TL;DR

This study reveals that UTe$_2$ is a fully gapped spin-triplet superconductor with topological properties, hosting Majorana surface states, challenging previous beliefs about its nodal structure.

Contribution

The paper conclusively determines the isotropic fully gapped pairing state in UTe$_2$, establishing it as a 3D topological superconductor with potential for hosting Majorana quasiparticles.

Findings

Zero-temperature thermal conductivity is vanishingly small, indicating no nodes in the gap.

Superconducting order parameter belongs to the isotropic $A_u$ representation.

UTe$_2$ is likely a 3D strong topological superconductor with helical Majorana surface states.

Abstract

Spin-triplet superconductors provide an ideal platform for realizing topological superconductivity with emergent Majorana quasiparticles. The promising candidate is the recently discovered superconductor UTe$ _2$, but the symmetry of the superconducting order parameter remains highly controversial. Here we determine the superconducting gap structure by the thermal conductivity of ultra-clean UTe$ _2$ single crystals. We find that the $a$ axis thermal conductivity divided by temperature $\kappa/T$ in zero-temperature limit is vanishingly small for both magnetic fields $\mathbf{H}||a$ and $\mathbf{H}||c$ axes up to $H/H_{c2}\sim 0.2$, demonstrating the absence of any types of nodes around $a$ axis contrary to the previous belief. The present results, combined with the reduction of the NMR Knight shift in the superconducting state, 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 (3D) strong topological superconductor characterized by a 3D winding number, hosting helical Majorana surface states on any crystal plane.