Observation of a neutron spin resonance in the bilayered superconductor CsCa 2 Fe 4 As 4 F 2 .

We report inelastic neutron scattering (INS) investigations on the bilayer Fe-based superconductor CsCa(2)Fe(4)As(4)F(2)above and below its superconducting transition temperatureT(c)approximate to 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 Fe(2)As(2)separated by insulating CaF(2)layers 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 (Q) at 5 K compared to 54 K, at an energy of 15 meV. The energyE(R)of the observed spin resonance is broadly consistent with the relationshipE(R)= 4.9k(B)T(c), 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 ofE(R)with that deduced from the total superconducting gap value integrated over the Fermi surface.