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

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.

14 pages, 16 figures