Low-energy spin excitations in Li(0.8)Fe(0.2)ODFeSe superconductor studied with inelastic neutron scattering

Abstract

We report an inelastic neutron scattering study of single crystals of (Li$_{0.8}$Fe$_{0.2}$)ODFeSe. Temperature-dependent low-energy spin excitations are observed near $\mathbf{Q}=$ (0.5, 0.27, 0.5) and equivalent wave vectors symmetrically surrounding $\mathbf{Q}=$ (0.5, 0.5, 0.5) in the 1-Fe Brillouin zone, consistent with a Fermi-surface-nesting description. The excitations are broadly distributed in energy, ranging from 16 to 35 meV. Upon cooling below the superconducting critical temperature ($T_\mathrm{c}$), magnetic response below twice the superconducting gap $2\Delta_\mathrm{SC}$ exhibits an abrupt enhancement, consistent with the notion of spin resonance, whereas the response at higher energies increases more gradually with only a weak anomaly at $T_\mathrm{c}$. Our results suggest that (Li$_{0.8}$Fe$_{0.2}$)ODFeSe might be on the verge of a crossover between different Cooper-pairing channels with distinct symmetries.