Abstract
We report inelastic neutron scattering measurements of low-energy spin fluctuations in ${\mathrm{FeSe}}_{0.95}{\mathrm{S}}_{0.05}$ (${T}_{c}=10$ K). Our experiments revealed a resonance mode at 3.5 meV, accompanied by a spin gap below 3 meV at the stripe wave vector in the superconducting state. The resonance mode is sharp in energy and appears at an energy level considerably lower than the superconducting gap, implying that the mode is a bound spin exciton below the superconducting gap rather than an enhanced paramagnon. An abrupt enhancement of stripe spin fluctuations is found below a nematic ordering temperature of ${T}_{s}=80$ K. These results indicate that the direct coupling between the stripe spin fluctuations, nematicity, and superconductivity persists in sulfur doped FeSe, where superconductivity is enhanced and nematicity is partially suppressed.
Published Version
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