Nodal Superconductivity of UTe$_2$ Probed by Field-Angle-Resolved Specific Heat on a Crystal with $T_{\rm c}=2.1$ K

TL;DR

This study uses field-angle-resolved specific heat measurements on UTe₂ to investigate its nodal superconducting gap structure, revealing anisotropic quasiparticle excitations aligned along the b axis, which informs its pairing symmetry.

Contribution

It provides experimental evidence for the nodal structure of UTe₂'s superconducting gap, supporting models with either point or line nodes consistent with specific symmetry classifications.

Findings

Linear field dependence of specific heat along the b axis at low temperatures.

Pronounced anisotropy indicating nodal quasiparticles aligned with the b axis.

Compatibility with theoretical models suggesting point or line nodes in the gap structure.

Abstract

Field-angle-resolved specific-heat measurements were performed on a clean single crystal of a spin-triplet superconductor UTe$_2$ with $T_{\rm c}=2.1$ K and a low residual electronic specific heat. At low temperatures, the specific heat exhibits a linear dependence on the magnetic field when the field is applied precisely along the $b$ axis, in stark contrast to its rapid increase at low fields for other orientations. This pronounced anisotropy suggests the presence of nodal quasiparticle excitations with the Fermi velocity predominantly aligned along the $b$ axis. Considering the characteristic field-angle dependences of both the specific heat and the upper critical field, these observations are broadly compatible with theoretical models that assume a superconducting gap structure featuring either point nodes consistent with $B_{\rm 2u}$ symmetry, allowed in the infinitely strong spin-orbit coupling scheme, or line nodes confined to flat regions of the quasi-two-dimensional Fermi surface, consistent with $^3B_{\rm 3u}$ symmetry in the finite spin-orbit classification scheme. These results yield crucial hints for resolving the pairing symmetry of UTe$_2$, paving the way for a deeper understanding of its spin-triplet superconductivity.