Neutron Spin Resonance in the Heavily Hole-Doped KFe2As2 Superconductor

Abstract

We report high-resolution neutron scattering measurements of the low energy spin fluctuations of KFe2As2, the end member of the hole-doped Ba1−xKxFe2As2 family with only hole pockets, above and below its superconducting transition temperature Tc (∼ 3.5 K). Our data reveals clear spin fluctuations at the incommensurate wave vector (0.5 ± δ, 0, L), (δ = 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 (2δ = 0.4) of the resonance mode (∼ 1.2 meV) is considerably larger than the previously reported value (2δ ≈ 0.32) at higher energies (≥∼ 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 KFe2As2. PACS numbers:

Enhanced critical temperatures in the strongly overdoped iron-based superconductors AFe2As2 (A = K, Cs, Rb) observed by point contacts

“…therein) and requires additional theoretical studies for the present case. In this context, the recent observation of a resonance mode in KFe2As2 strongly supports the unconventional nature of superconductivity [25] although several details of the interpretation remain in our opinion unclear as explained in detail elsewhere.…”