Spin anisotropy of the resonance peak in superconducting FeSe0.5Te0.5

We have used polarized-neutron inelastic scattering to resolve the spin fluctuations in superconducting FeSe0.5Te0.5 into components parallel and perpendicular to the layers. A spin resonance at an energy of 6.5 meV is observed to develop below T-c in both fluctuation components. The resonance peak is anisotropic, with the in-plane component slightly larger than the out-of-plane component. Away from the resonance peak, the magnetic fluctuations are isotropic in the energy range studied. The results are consistent with a dominant singlet pairing state with s(+/-) symmetry, with a possible minority component of different symmetry.