Superconducting Properties of Heavy Fermion UTe$_2$ Revealed by $^{125}$Te-Nuclear Magnetic Resonance

TL;DR

This study uses $^{125}$Te-NMR to investigate the superconducting properties of UTe$_2$, revealing evidence of unconventional, likely spin-triplet, superconductivity with a two-gap structure and minimal change in spin susceptibility below $T_c$.

Contribution

First NMR study on UTe$_2$ demonstrating its unconventional, likely spin-triplet, superconductivity with a two-gap model and detailed spin susceptibility behavior.

Findings

$1/T_1T$ decreases below $T_c$ without a coherence peak.

Superconductivity explained by a two-gap model.

Small decrease in Knight shift suggests spin-triplet pairing.

Abstract

We have performed the $^{125}$Te-nuclear magnetic resonance (NMR) measurement in the field along the $b$ axis on the newly discovered superconductor UTe$_2$, which is a candidate of a spin-triplet superconductor. The nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ abruptly decreases below a superconducting (SC) transition temperature $T_c$ without showing a coherence peak, indicative of UTe$_2$ being an unconventional superconductor. It was found that the temperature dependence of $1/T_1T$ in the SC state cannot be understood by a single SC gap behavior but can be explained by a two SC gap model. The Knight shift, proportional to the spin susceptibility, decreases below $T_c$, but the magnitude of the decrease is much smaller than the decrease expected in the spin-singlet pairing. Rather, the small Knight-shift decrease as well as the absence of the Pauli-depairing effect can be interpreted by the spin triplet scenario.