Pseudo Point Nodal Superconducting Gap in Spin-Triplet UTe$_2$

TL;DR

This study reveals that UTe$_2$ is a fully gapped spin-triplet superconductor with pseudo point nodes, providing new insights into its pairing mechanism and topological nature through high-resolution thermal conductivity measurements.

Contribution

The paper demonstrates that UTe$_2$ has a pseudo point-nodal gap structure, resolving previous conflicting reports and advancing understanding of its topological superconductivity.

Findings

UTe$_2$ exhibits negligible residual thermal conductivity at zero temperature.

A threshold magnetic field indicates a change in quasiparticle transport behavior.

The minimal superconducting gap along the b-axis is approximately 10% of the maximum gap.

Abstract

The unconventional superconductor UTe$_2$ represents a rare example of spin-triplet pairing with potentially topologically protected quantum states. However, conflicting reports on its gap structure, particularly regarding point nodes, have hindered understanding of the order parameter symmetry and topological properties. Here we report high-resolution thermal conductivity measurements on high-quality UTe$_2$ single crystals down to ~50 mK that resolve the gap anisotropy through bulk directional transport. The $b$-axis thermal conductivity $\kappa_b/T$ exhibits negligible residual conductivity as $T \to 0$, and its temperature dependence is consistent with a small superconducting energy gap along the $b$-axis. Under magnetic fields, the residual $\kappa_b/T$ shows only weak field-induced enhancement. Remarkably, a threshold field emerges at low fields for $H \parallel a$, characterized by a kink that signals a change in quasiparticle transport normal to the field. Below the threshold, $\kappa_b/T$ remains isotropic for all field orientations, whereas strong anisotropy between transport along and normal to the field develops above it. These signatures strongly suggest that UTe$_2$ exhibits a fully gapped state with a pseudo point-nodal structure, where gap minima approach but never reach zero. We estimate the minimal gap $\Delta_{min}/\Delta_0 \sim 0.1$ along the $b$-axis, where $\Delta_0$ is the characteristic superconducting gap. This unusual gap structure provides crucial insights into the pairing mechanism and topology of this spin-triplet superconductor and excludes non-unitary mixing of pairing symmetries.