Temperature and polarization dependence of low-energy magnetic fluctuations in nearly optimally doped NaFe0.9785Co0.0215As

We use unpolarized and polarized neutron scattering to study the temperature and polarization dependence of low-energy magnetic fluctuations in nearly optimally doped ${\mathrm{NaFe}}_0.9785{\mathrm{Co}}_0.0215\mathrm{As}$, with coexisting superconductivity $({T}_{\mathrm{c}}\ensuremath{\approx}19$ K) and weak antiferromagnetic order $({T}_{\mathrm{N}}\ensuremath{\approx}30$ K, ordered moment $\ensuremath{\approx}0.02\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Fe})$. A single spin resonance mode with intensity tracking the superconducting order parameter is observed, although energy of the mode only softens slightly upon approaching ${T}_{\mathrm{c}}$. Polarized neutron scattering reveals that the single resonance is mostly isotropic in spin space, similar to overdoped ${\mathrm{NaFe}}_0.935{\mathrm{Co}}_0.045\mathrm{As}$ but different from optimal electron-, hole-, and isovalently doped ${\mathrm{BaFe}}₂{\mathrm{As}}₂$ compounds, all featuring an additional prominent anisotropic component. Spin anisotropy in ${\mathrm{NaFe}}_0.9785{\mathrm{Co}}_0.0215\mathrm{As}$ is instead present at energies below the resonance, which becomes partially gapped below ${T}_{\mathrm{c}}$, similar to the situation in optimally doped ${\mathrm{YBa}}₂{\mathrm{Cu}}₃{\mathrm{O}}_6.9$. Our results indicate that anisotropic spin fluctuations in ${\mathrm{NaFe}}_{1\ensuremath{-}x}{\mathrm{Co}}_x\mathrm{As}$ appear in the form of a resonance in the underdoped regime, become partially gapped below ${T}_{\mathrm{c}}$ near optimal doping, and disappear in overdoped compounds.