Enhancement of antiferromagnetic spin fluctuations in UTe$_2$ under pressure revealed by $^{125}$Te NMR

TL;DR

This study uses $^{125}$Te NMR to reveal that antiferromagnetic spin fluctuations in UTe$_2$ increase under pressure, providing insights into their role in the material's two distinct superconducting phases.

Contribution

The paper demonstrates the enhancement of antiferromagnetic spin fluctuations with pressure in UTe$_2$, linking these fluctuations to the second superconducting phase, which is a novel insight.

Findings

AFM spin fluctuations increase with pressure

FM fluctuations favor the first SC phase

AFM fluctuations are crucial for the second SC phase

Abstract

Characterizing magnetic fluctuations is one of the keys to understanding the origin of superconductivity in the spin-triplet superconductor UTe$_2$ which exhibits two superconducting (SC) phases (SC1 and SC2) under pressure: SC1 where a superconducting transition temperature of $T_{\rm c}$ decreases with pressure while $T_{\rm c}$ of SC2 rises with pressure. Previously, D. Ambika et al. [Phys. Rev. B 105, L220403 (2022)] have reported the possible coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) spin fluctuations in UTe$_2$ under pressure from their nuclear magnetic resonance (NMR) measurements. To delve the relationship between the magnetic fluctuations and the two SC phases, we have carried out detailed $^{125}$Te NMR measurements on a single crystal of UTe$_2$ with $T_{\rm c}$ = 1.6 K at various pressures ranging from 0 to 2.05 GPa. By comparing the temperature $T$ dependence of nuclear spin-lattice relaxation rates divided by temperature 1/$T_1T$ with that of the Knight shift $K$ for magnetic fields along the $a$, $b$, and $c$ directions, we evidence the enhancement of AFM spin fluctuations with increasing pressure. Based on the results, we suggest that FM spin fluctuations are more favorable for SC1 and AFM spin fluctuations are crucial for SC2. Our findings will inspire further study on this material to understand the peculiar SC phases in detail.