Structure of spin excitations in heavily electron-doped Li(0.8)Fe(0.2)ODFeSe superconductors

Heavily electron-doped iron-selenide high-transition-temperature (high-T (c)) superconductors, which have no hole Fermi pockets, but have a notably high T (c), have challenged the prevailing s (±) pairing scenario originally proposed for iron pnictides containing both electron and hole pockets. The microscopic mechanism underlying the enhanced superconductivity in heavily electron-doped iron-selenide remains unclear. Here, we used neutron scattering to study the spin excitations of the heavily electron-doped iron-selenide material Li(0.8)Fe(0.2)ODFeSe (T (c) = 41 K). Our data revealed nearly ring-shaped magnetic resonant excitations surrounding (π, π) at ∼21 meV. As the energy increased, the spin excitations assumed a diamond shape, and they dispersed outward until the energy reached ∼60 meV and then inward at higher energies. The observed energy-dependent momentum structure and twisted dispersion of spin excitations near (π, π) are analogous to those of hole-doped cuprates in several aspects, thus implying that such spin excitations are essential for the remarkably high T (c) in these materials.