Clear Reduction in Spin Susceptibility and Superconducting Spin Rotation for $H \parallel a$ in the Early-Stage Sample of Spin-Triplet Superconductor UTe$_2$

TL;DR

This study re-measured the $a$-axis spin susceptibility in UTe$_2$, revealing a clear reduction in susceptibility below $T_{SC}$ and demonstrating suppression of this effect with increasing magnetic field, clarifying previous inconsistent findings.

Contribution

It provides the first accurate measurement of the $a$-axis spin susceptibility in early-stage UTe$_2$, confirming the reduction in susceptibility in the superconducting state and explaining prior discrepancies.

Findings

$a$-axis spin susceptibility decreases by ~3% below $T_{SC}$

Reduction in susceptibility is suppressed at around 1.5 T due to spin rotation

Previous absence of Knight shift reduction was due to measurement issues

Abstract

We report the re-measurement of the $a$-axis spin susceptibility component in an early-stage sample of the spin-triplet superconductor UTe$_2$ with the transition temperature of $T_{\rm SC}$ = 1.6 K. Using Knight-shift measurements along the $b$ axis and at a 10-degree tilt from the $b$ axis towards the $a$ axis, we accurately determined the $a$-axis component without directly measuring the $a$-axis Knight shift. Our results reveal a decrease of approximately 3\% in the $a$-axis spin susceptibility in the superconducting state under $a$-axis magnetic field $\mu_0 H_a \sim 0.1$ T, indicating that the spin susceptibility decreases similarly in both early-stage and ultraclean samples with $T_{\rm SC}$ = 2.1 K. The previously reported absence of the reduction in Knight shift is attributed to the missing of signal from the superconducting region and to the detection of residual signals from the non-superconducting region instead. We also found that the decrease in the $a$-axis spin susceptibility is immediately suppressed with increasing the $a$-axis magnetic field and is estimated to be completely suppressed at around 1.5 T due to superconducting spin rotation.