Pulse Dependence of the Nuclear Spin-Spin Relaxation 1/T₂ in the Normal State of UTe₂

UTe₂ is a recently discovered heavy-fermion superconductor that exhibits properties consistent with an odd-parity superconducting state [1]. Initial muon spin relaxation and nuclear magnetic resonance (NMR) measurements on chemical-vapor-transport (CVT) grown samples were interpreted as evidence for ferromagnetic (FM) spin-fluctuation mediated superconductivity [2,3], whereas neutron scattering measurements have detected antiferromagnetic spin fluctuations [4]. Here we use ¹²⁵Te NMR in powdered and single crystal UTe₂ to probe the nature of these fluctuations. The spin-spin relaxation rate (1/T₂) is observed to be dependent on the details of the NMR pulse sequence. In particular, an apparent maximum in the rate 1/T₂ at T_m ≈ 10 K (for a CVT-grown single crystal) is reduced for smaller pulse amplitudes. The peak in 1/T₂ otherwise correlates with the field and temperature dependence of the skin depth. This maximum, reportedly evidence for FM spin fluctuations [3], is not observed in studies of powdered UTe₂.

[1] Matsumura et al, J. Phys. Soc. Jpn. 92, 063701 (2023)

[2] Sundar et al, Phys. Rev. B 100, 140502(R) (2019)

[3] Tokunaga et al, J. Phys. Soc. Jpn. 91, 023707 (2022)

[4] Knafo et al, Phys. Rev. B 104, L100409 (2021)