Resonance from antiferromagnetic spin fluctuations for superconductivity in UTe$_2$

Abstract

Superconductivity originates from the formation of bound (Cooper) pairs of electrons that can move through the lattice without resistance below the superconducting transition temperature $T_c$. Electron Cooper pairs in most superconductors form anti-parallel spin singlets with total spin $S=0$, although 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 quantum computation. Because spin-triplet pairing is usually mediated by ferromagnetic (FM) spin fluctuations, uranium based materials near an FM instability are considered to be ideal candidates for realizing spin-triplet superconductivity. Indeed, UTe$_2$, which has a $T_c\approx 1.6$ K, has been identified as a candidate for a chiral spin-triplet topological superconductor near an FM instability, although it also has antiferromagnetic (AF) spin fluctuations. Here we use inelastic neutron scattering (INS) to show that superconductivity in UTe$_2$ is coupled to a sharp magnetic excitation, termed resonance, at the Brillouin zone boundary near AF order. Because the resonance has only been found in spin-singlet unconventional superconductors near an AF instability, its observation in UTe$_2$ suggests that AF spin fluctuations may also induce spin-triplet pairing or that electron pairing in UTe$_2$ has a spin-singlet component.