Resonance from antiferromagnetic spin fluctuations for spin-triplet superconductivity in UTe2*

Superconductivity has its universal origin in the formation of bound (Cooper) pairs of electrons that can move through the lattice without resistance below the superconducting transition temperature Tc. While electron Cooper pairs in most superconductors form anti-parallel spin-singlets with total spin S = 0, they can also form parallel spin-triplet Cooper pairs with S = 1 and an odd parity wavefunction. Spin-triplet pairing is important because it can host topological states and Majorana fermions relevant for fault tolerant quantum computation. However, spin-triplet pairing has not been unambiguously identified in any solid state systems. Since spin-triplet pairing is usually mediated by ferromagnetic (FM) spin fluctuations, uranium based heavy-fermion materials near a FM instability are considered ideal candidates for realizing spin-triplet superconductivity. Indeed, UTe2, which has a Tc _ 1.6 K, has been identified as a candidate for chiral spin-triplet topological superconductor near a FM instability, although the system also exhibits antiferromagnetic (AF) spin fluctuations. Here we use inelastic neutron scattering (INS) to show that superconductivity in UTe2 is coupled with a sharp magnetic excitation, termed resonance, at the Brillouin zone (BZ) boundary near AF order. Since the resonance has only been found in spin-singlet unconventional superconductors near an AF instability, its discovery in UTe2 suggests that AF spin fluctuations may also induce spin-triplet pairing or that electron pairing in UTe2 has a spin-singlet component.