Neutron Spin Resonance near a Lifshitz Transition in Overdoped Ba0.4K0.6Fe2As2

Elucidating the relationship between spin excitations and fermiology is essential for clarifying the pairing mechanism in iron-based superconductors (FeSCs). Here, we report inelastic neutron scattering results on the hole overdoped Ba0.4K0.6Fe2As2 near a Lifshitz transition, where the electron pocket at M point is nearly replaced by four hole pockets. In the normal state, the spin excitations is observed at incommensurate wave vectors with a chimney-like dispersion. By cooling down to the superconducting state, a neutron spin resonance mode emerges with a peak energy of Er = 14–15 meV, weakly modulated along the L-direction. The incommensurability notably increases at low energies, giving rise to downward dispersions of the resonance mode. This behavior contrasts sharply with the upward dispersions of resonance observed in optimally doped Ba0.67K0.33Fe2As2 contributed by the hole to electron scattering, but resembles those in KFe2As2 and KCa2Fe4As4F2 where the fermiology is dominated by hole pockets. These results highlight the critical role of electronic structure modifications near the Fermi level, especially in governing interband scattering under imperfect nesting conditions, which fundamentally shape the spin dynamics of FeSCs.