Resonance from antiferromagnetic spin fluctuations for superconductivity in UTe2

Superconductivity originates from the formation of bound (Cooper) pairs of electrons that can move through the lattice without resistance below the superconducting transition temperature T c (ref. 1 ). Electron Cooper pairs in most superconductors form anti-parallel spin singlets with total spin S = 0 (ref. 2 ), although they can also form parallel spin-triplet Cooper pairs with S = 1 and an odd parity wavefunction 3 . Spin-triplet pairing is important because it can host topological states and Majorana fermions relevant for quantum computation 4 , 5 . Because spin-triplet pairing is usually mediated by ferromagnetic (FM) spin fluctuations 3 , uranium-based materials near an FM instability are considered to be ideal candidates for realizing spin-triplet superconductivity 6 . Indeed, UTe 2 , which has a T c ≈ 1.6 K (refs. 7 , 8 ), has been identified as a candidate for a chiral spin-triplet topological superconductor near an FM instability 7 – 14 , although it also has antiferromagnetic (AF) spin fluctuations 15 , 16 . Here we use inelastic neutron scattering (INS) to show that superconductivity in UTe 2 is coupled to a sharp magnetic excitation, termed resonance 17 – 23 , at the Brillouin zone boundary near AF order. Because the resonance has only been found in spin-singlet unconventional superconductors near an AF instability 17 – 23 , its observation in UTe 2 suggests that AF spin fluctuations may also induce spin-triplet pairing 24 or that electron pairing in UTe 2 has a spin-singlet component. Inelastic neutron scattering measurements show that superconductivity in UTe 2 is associated with a resonance near antiferromagnetic order that suggests an unexpected spin-singlet component to the electron pairing.