Electron doping evolution of the magnetic excitations in NaFe$_{1-x}$Co$_x$As

TL;DR

This study investigates how electron doping via Co substitution affects magnetic excitations in NaFeAs, revealing that low-energy excitations are suppressed and broadened with doping, while high-energy excitations remain largely unaffected, highlighting their different origins.

Contribution

It provides a detailed doping-dependent analysis of magnetic excitations in NaFe$_{1-x}$Co$_x$As using neutron scattering, emphasizing the distinct behaviors of low- and high-energy spin excitations.

Findings

Low-energy spin excitations are suppressed and broadened with doping.

High-energy spin excitations are weakly dependent on Co doping.

Total fluctuating moment decreases systematically with doping.

Abstract

We use time-of-flight (ToF) inelastic neutron scattering (INS) spectroscopy to investigate the doping dependence of magnetic excitations across the phase diagram of NaFe$_{1-x}$Co$_x$As with $x=0, 0.0175, 0.0215, 0.05,$ and $0.11$. The effect of electron-doping by partially substituting Fe by Co is to form resonances that couple with superconductivity, broaden and suppress low energy ($E\le 80$ meV) spin excitations compared with spin waves in undoped NaFeAs. However, high energy ($E> 80$ meV) spin excitations are weakly Co-doping dependent. Integration of the local spin dynamic susceptibility $\chi^{\prime\prime}(\omega)$ of NaFe$_{1-x}$Co$_x$As reveals a total fluctuating moment of 3.6 $\mu_B^2$/Fe and a small but systematic reduction with electron doping. The presence of a large spin gap in the Co-overdoped nonsuperconducting NaFe$_{0.89}$Co$_{0.11}$As suggests that Fermi surface nesting is responsible for low-energy spin excitations. These results parallel Ni-doping evolution of spin excitations in BaFe$_{2-x}$Ni$_x$As$_2$, confirming the notion that low-energy spin excitations coupling with itinerant electrons are important for superconductivity, while weakly doping dependent high-energy spin excitations result from localized moments.