$b$-axis and $c$-axis Knight shift measurements in the superconducting state on ultraclean UTe$_2$ with $T_c$ = 2.1 K

TL;DR

This study measures the Knight shifts along the b and c axes in ultraclean UTe2 to determine the d-vector components of its spin-triplet superconducting order parameter, revealing it has components along all three axes.

Contribution

It provides the first detailed Knight shift measurements along multiple axes in ultraclean UTe2, confirming the d-vector has components in all three crystal directions.

Findings

Spin susceptibility decreases along b and c axes in the superconducting state.

The d-vector has components along all three crystal axes.

Knight shifts are consistent with spin-triplet pairing.

Abstract

Knight shifts along the $b$ and $c$ axes ($K_b$ and $K_c$) at two crystallographically distinct Te sites were measured down to 70 mK using $^{125}$Te nuclear magnetic resonance (NMR) on an ultraclean UTe$_2$ single crystal with a superconducting (SC) transition temperature $T_{\mathrm{c}}$ = 2.1 K. This was carried out to determine the $\boldsymbol{d}$-vector components, which are the order parameter in the spin-triplet pairing. Although the decrease in $K_b$ and $K_c$ is comparable to the theoretical estimation of the SC diamagnetic shielding effect, it is confirmed, by taking the difference between two Knight shifts at the distinct Te sites, that the spin susceptibility along the $b$ and $c$ axes decreases in the SC state. Taking into account the large decrease in $K_a$ in the SC state, we conclude that the $\boldsymbol{d}$ vector has components along all three crystal axes.