Observation of a neutron spin resonance in the bilayered superconductor CsCa₂Fe₄As₄F₂

Abstract

We report inelastic neutron scattering (INS) investigations on the bilayer Fe-based superconductor CsCa₂Fe₄As₄F₂above and below its superconducting transition temperature<i>T</i>_c≈ 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 Fe₂As₂separated by insulating CaF₂layers 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 (<i>Q</i>) at 5 K compared to 54 K, at an energy of 15 meV. The energy<i>E</i>_Rof the observed spin resonance is broadly consistent with the relationship<i>E</i>_R= 4.9<i>k</i>_B<i>T</i>_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 of<i>E</i>_Rwith that deduced from the total superconducting gap value integrated over the Fermi surface.