Neutron Spin Resonance in the Heavily Hole-Doped KFe2As2 Superconductor

We report high-resolution neutron scattering measurements of the low energy spin fluctuations of KFe_{2}As_{2}, the end member of the hole-doped Ba_{1-x}K_{x}Fe_{2}As_{2} family with only hole pockets, above and below its superconducting transition temperature T_{c} ( approximately 3.5 K). Our data reveal clear spin fluctuations at the incommensurate wave vector (0.5+/-delta, 0, L), (delta=0.2) (1-Fe unit cell), which exhibit L-modulation peaking at L=0.5. Upon cooling to the superconducting state, the incommensurate spin fluctuations gradually open a spin gap and form a sharp spin resonance mode. The incommensurability (2delta=0.4) of the resonance mode ( approximately 1.2 meV) is considerably larger than the previously reported value (2delta approximately 0.32) at higher energies (>/= approximately 6 meV). The determination of the momentum structure of spin fluctuation in the low energy limit allows a direct comparison with the realistic Fermi surface and superconducting gap structure. Our results point to an s-wave pairing with a reversed sign between the hole pockets near the zone center in KFe_{2}As_{2}.