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

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.