Dispersion of neutron spin resonance mode in Ba0.67K0.33Fe2As2 *

Abstract

We report an inelastic neutron scattering investigation on the spin resonance mode in the optimally hole-doped iron-based superconductor Ba0.67K0.33Fe2As2 with Tc= 38.2 K. Although the resonance is nearly two-dimensional with peak energy E R ≈ 14 meV, it splits into two incommensurate peaks along the longitudinal direction ([H,0,0]) and shows an upward dispersion persisting to 26 meV. Such dispersion breaks through the limit of total superconducting gaps Δ tot = |Δk | + |Δ k+Q | (about 11–17 meV) on nested Fermi surfaces measured by high resolution angle resolved photoemission spectroscopy (ARPES). These results cannot be fully understood by the magnetic exciton scenario under s±-pairing symmetry of superconductivity, and suggest that the spin resonance may not be restricted by the superconducting gaps in the multi-band systems.