Dispersion of neutron spin resonance mode in Ba0.67K0.33Fe2As2 * *Project supported by the National Key Research and Development Program of China (Grant Nos. 2018YFA0704200, 2018YFA0305602, 2017YFA0303100, and 2017YFA0302900), the National Natural Science Foundation of China (Grant Nos. 11822411 and 11961160699), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (CAS) (Grant Nos. XDB25000000 and XDB07020300), K.C.Wong Education

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.