Temperature and polarization dependence of low-energy magnetic fluctuations in nearly-optimal-doped NaFe$_{0.9785}$Co$_{0.0215}$As

TL;DR

This study investigates how low-energy magnetic fluctuations in nearly-optimal-doped NaFeCoAs vary with temperature and polarization, revealing a mostly isotropic resonance that becomes gapped below the superconducting transition.

Contribution

It provides the first polarized neutron scattering analysis of magnetic fluctuations in nearly-optimal-doped NaFeCoAs, highlighting the evolution of spin anisotropy across doping levels.

Findings

A single spin resonance mode tracks superconductivity.

Resonance is mostly isotropic in spin space.

Spin anisotropy appears below the resonance at energies below it.

Abstract

We use unpolarized and polarized neutron scattering to study the temperature and polarization dependence of low-energy magnetic fluctuations in nearly-optimal-doped NaFe$_{0.9785}$Co$_{0.0215}$As, with coexisting superconductivity ($T_{\rm c}\approx19$ K) and weak antiferromagnetic order ($T_{\rm N}\approx30$ K, ordered moment $\approx0.02$ $\mu_{\rm B}$/Fe). A single spin resonance mode with intensity tracking the superconducting order parameter is observed, although energy of the mode only softens slightly on approaching $T_{\rm c}$. Polarized neutron scattering reveals that the single resonance is mostly isotropic in spin space, similar to overdoped NaFe$_{0.935}$Co$_{0.045}$As but different from optimal electron-, hole-, and isovalent-doped BaFe$_2$As$_2$ compounds, all featuring an additional prominent anisotropic component. Spin anisotropy in NaFe$_{0.9785}$Co$_{0.0215}$As is instead present at energies below the resonance, which becomes partially gapped below $T_{\rm c}$, similar to the situation in optimal-doped YBa$_2$Cu$_3$O$_{6.9}$. Our results indicate that anisotropic spin fluctuations in NaFe$_{1-x}$Co$_x$As appear in the form of a resonance in the underdoped regime, become partially gapped below $T_{\rm c}$ near optimal doping and disappear in overdoped compounds.