Study of drastic changes in spin fluctuations of UTe₂ under pressure using ¹²⁵Te NMR

The heavy-fermion superconductor UTe₂, characterized by a superconducting (SC) transition temperature (T_c) ranging from approximately 1.6 to 2.0 K, is being investigated for its distinct SC and magnetic properties. Under pressure, UTe2 (with T_c ~ 1.6 K) exhibits two superconducting phases: SC1, which has a lower Tc and is suppressed with increasing pressure, and SC2, with T_c, which peaks at about 3 K around 1.2 GPa before rapidly declining and vanishing at approximately 1.5 GPa. A magnetically ordered state emerges beyond this critical pressure (p_c ~ 1.5 GPa). Nuclear magnetic resonance (NMR) measurements at ambient pressure and under pressure suggest the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) spin fluctuations in UTe₂. To investigate the evolution of spin fluctuations in UTe₂ under pressure, we employed ¹²⁵Te NMR measurements on ¹²⁵Te enriched UTe₂ single crystal with T_c ~ 1.6 K. We report on the temperature (1.5 -150 K) and pressure (0 - 1.9 GPa) dependence of the Knight shift (K) and the nuclear spin-lattice relaxation rate (1/T1) with the magnetic field (H) oriented parallel to all three crystalline axes. Using 1/T1T measurements for fields aligned along the a, b, and c directions, we probed the spin fluctuations. We compared it with static susceptibility data represented by K to understand the nature of these fluctuations better. Our observations indicate significant changes in spin fluctuations above and below the critical pressure.