Anisotropic response of spin susceptibility in the superconducting state of UTe$_2$ probed with $^{125}$Te-NMR measurement

TL;DR

This study uses $^{125}$Te-NMR to reveal anisotropic spin susceptibility behavior in the superconducting state of UTe$_2$, supporting spin-triplet pairing and suggesting a change in the $d$-vector orientation at high magnetic fields.

Contribution

It provides the first detailed measurement of anisotropic spin susceptibility in UTe$_2$'s superconducting state using NMR, highlighting the complex $H$-dependent behavior.

Findings

Spin susceptibilities along $b$ and $c$ axes slightly decrease in the SC state.

The $K_c$ decrease diminishes with increasing magnetic field.

The $d$-vector orientation likely changes above 13 T.

Abstract

To investigate spin susceptibility in a superconducting (SC) state, we measured the $^{125}$Te-nuclear magnetic resonance (NMR) Knight shifts at magnetic fields ($H$) up to 6.5 T along the $b$ and $c$ axes of single-crystal UTe$_2$, a promising candidate for a spin-triplet superconductor. In the SC state, the Knight shifts along the $b$ and $c$ axes ($K_b$ and $K_c$, respectively) decreased slightly and the decrease in $K_b$ was almost constant up to 6.5 T. The reduction in $K_c$ decreased with increasing $H$, and $K_c$ was unchanged through the SC transition temperature at 5.5 T, excluding the possibility of spin-singlet pairing. Our results indicate that spin susceptibilities along the $b$ and $c$ axes slightly decrease in the SC state in low $H$, and the $H$ response of SC spin susceptibility is anisotropic in the $bc$ plane. We discuss the possible $d$-vector state within the spin-triplet scenario and suggest that the dominant $d$-vector component for the case of $H \parallel b$ changes above 13 T, where $T_{\rm c}$ increases with increasing $H$.