Low-energy spin excitations in $({\mathbf{Li}}_{0.8}{\mathbf{Fe}}_{0.2})\text{ODFeSe}$ superconductor studied with inelastic neutron scattering

We report an inelastic neutron scattering study of single crystals of (${\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}$)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 ($T_{\mathrm{c}}$), a magnetic response below twice the superconducting gap $2{\mathrm{\Delta }}_{\mathrm{SC}}$ 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 (${\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}$)ODFeSe might be on the verge of a crossover between different Cooper-pairing channels with distinct symmetries.

Figure 1

Temperature dependence of magnetic susceptibility of (${\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}$)ODFeSe measured with a magnetic field of 10 Oe applied perpendicular to the $c$ axis after zero-field cooling. Left inset: Photograph of our coaligned sample for INS experiments. Right inset: X-ray Laue-reflection pattern from a single crystal (x rays incident along the $c$ axis).

Figure 2

(a) $\mathbf{Q}$ scans along the $(0.5,K,0.5)$ direction at a fixed energy transfer of 18 meV and temperatures of 4 and 42 K. (b) Intensity difference between the two temperatures. (c) $\mathbf{Q}$ scans at 21 meV in a direction perpendicular to that in (a). (d) The intensity difference between the two temperatures in (c). Arrows indicate the fitted peak centers.

Figure 3

(a) Energy scans at fixed $\mathbf{Q}$ positions $(0.5,0.68,0.5)$ and $(0.5,0.73,0.5)$ in the superconducting state ($T=4$ K) and normal state ($T=42$ K). (b) Intensity difference between the two temperatures.

Figure 4

(a) $T$ dependence of INS signal at $\mathbf{Q}=(0.5,0.27,0.5)$ and 21 meV. The solid and open circles represent two sets of data obtained in separate continuous runs using the same experimental configuration. (b) $T$ dependence of INS signal at $\mathbf{Q}=(0.5,0.73,0.5)$ and 31 meV. Averaged intensities just above $T_{\mathrm{c}}$ are set to zero. Solid lines are a guide to the eye.