Observation of a neutron spin resonance in the bilayered superconductor CsCaFeAsF.

We report inelastic neutron scattering (INS) investigations on the bilayer Fe-based superconductor CsCaFeAsFabove and below its superconducting transition temperature≈ 28.9 K to investigate the presence of a neutron spin resonance. This compound crystallises in a body-centred tetragonal lattice containing asymmetric double layers of FeAsseparated by insulating CaFlayers and is known to be highly anisotropic. Our INS study clearly reveals the presence of a neutron spin resonance that exhibits higher intensity at lower momentum transfer () at 5 K compared to 54 K, at an energy of 15 meV. The energyof the observed spin resonance is broadly consistent with the relationship= 4.9, but is slightly enhanced compared to the values observed in other Fe-based superconductors. We discuss the nature of the electron pairing symmetry by comparing the value ofwith that deduced from the total superconducting gap value integrated over the Fermi surface.