Resonance from antiferromagnetic spin fluctuations for superconductivity in UTe 2 .

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.