Spin resonance in the superconducting state of Li${}_{1-x}$Fe${}_x$ODFe${}_{1-y}$Se observed by neutron spectroscopy

We have performed inelastic neutron scattering measurements on a powder sample of the superconductor lithium iron selenide hydroxide Li${}_{1-x}$Fe${}_x$ODFe${}_{1-y}$Se ($x\simeq 0.16,y\simeq 0.02$, $T_{\rm c} = 41$K)\mathrm{.}Thespectrumshowsanenhancedintensitybelow$T_{\rm c}$ over an energy range $0.64\times 2\mathrm{\Delta }<E<2\mathrm{\Delta }$, where $\mathrm{\Delta }$ is the superconducting gap, with maxima at the wave vectors $Q_1 \simeq 1.46$\mathring{A}$^{-1}$and$Q_2 \simeq 1.97$\mathring{A}$^{-1}$. The behavior of this feature is consistent with the spin resonance mode found in other unconventional superconductors, and strongly resembles the spin resonance observed in the spectrum of the molecular-intercalated iron selenide, Li${}_{0.6}$(ND${}_2$)${}_{0.2}$(ND${}_3$)${}_{0.8}$Fe${}_2$Se${}_2$. The signal can be described with a characteristic two-dimensional wave vector $(\pi ,0.67\pi )$ in the Brillouin zone of the iron square lattice, consistent with the nesting vector between electron Fermi sheets.