Electron doping evolution of the magnetic excitations in NaFe1−xCoxAs

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 ≤ 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 χ ' ' ( ω ) of NaFe 1 − x Co x As reveals a total fluctuating moment of 3.6 μ 2 B /Fe and a small but systematic reduction with electron doping. The presence of a large spin gap in 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 the Ni-doping evolution of spin excitations in BaFe 2 − x Ni x As 2 in spite of the differences in crystal structures and Fermi surface evolution in these two families of iron pnictides, thus 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