Electron doping evolution of the magnetic excitations in BaFe2−xNixAs2

Abstract

We use inelastic neutron scattering (INS) spectroscopy to study the magnetic excitations spectra throughout the Brillouin zone in electron-doped iron pnictide superconductors BaFe${}_{2\ensuremath{-}x}$Ni${}_{x}$As${}_{2}$ with $x=0.096,0.15,0.18$. While the $x=0.096$ sample is near optimal superconductivity with ${T}_{c}=20$ K and has coexisting static incommensurate magnetic order, the $x=0.15,0.18$ samples are electron overdoped with reduced ${T}_{c}$ of 14 and 8 K, respectively, and have no static antiferromagnetic (AF) order. In previous INS work on undoped ($x=0$) and electron optimally doped ($x=0.1$) samples, the effect of electron doping was found to modify spin waves in the parent compound BaFe${}_{2}$As${}_{2}$ below $\ensuremath{\sim}$100 meV and induce a neutron spin resonance at the commensurate AF ordering wave vector that couples with superconductivity. While the new data collected on the $x=0.096$ sample confirm the overall features of the earlier work, our careful temperature dependent study of the resonance reveals that the resonance suddenly changes its $Q$ width below ${T}_{c}$ similar to that of the optimally hole-doped iron pnictides Ba${}_{0.67}$K${}_{0.33}$Fe${}_{2}$As${}_{2}$. In addition, we establish the dispersion of the resonance and find it to change from commensurate to transversely incommensurate with increasing energy. Upon further electron doping to overdoped iron pnictides with $x=0.15$ and 0.18, the resonance becomes weaker and transversely incommensurate at all energies, while spin excitations above $\ensuremath{\sim}$100 meV are still not much affected. Our absolute spin excitation intensity measurements throughout the Brillouin zone for $x=0.096,0.15,0.18$ confirm the notion that the low-energy spin excitation coupling with itinerant electron is important for superconductivity in these materials, even though the high-energy spin excitations are weakly doping dependent.