Evolution of spin fluctuations in UTe₂ studied by ¹²⁵Te NMR under pressure

The superconductivity and magnetism in the newly discovered uranium-based superconductor UTe₂ have recently been a hot research topic. Under pressure, UTe₂ (with T_c = 1.6 K) shows two superconducting (SC) phases above 0.25 GPa. The lower T_c SC phase is suppressed with the increase in pressure, and the higher T_c phase is enhanced and takes a maximum T_c ∼ 3 K at around 1.2 GPa and is suppressed rapidly at higher pressures. A magnetically ordered state appears above critical pressure (p_c ∼ 1.5 GPa). Previous muon spin relaxation and nuclear magnetic resonance (NMR) measurements at ambient pressure have suggested the importance of the ferromagnetic spin fluctuations along the a-axis for the appearance of the spin-triplet superconducting state in UTe₂. The NMR technique can be used to investigate the evolution of spin fluctuations in UTe₂ by tuning the pressure in a wide range. In this study, we carried out ¹²⁵Te NMR measurements on ¹²⁵Te-enriched UTe₂ single crystals under pressure up to 2 GPa. We will report the temperature and pressure dependence of the Knight shift K and the nuclear spin-lattice relaxation rate 1/T₁ under the magnetic field H parallel to the b axis (H ∥ b) and H parallel to the c axis (H ∥ c). Based on the NMR data, we will discuss the nature of the spin fluctuations under pressure in UTe₂.