Polarized neutron scattering studies of magnetic excitations in electron-overdoped superconducting BaFe${}_{1.85}$Ni${}_{0.15}$As${}_2$

Abstract

We use polarized inelastic neutron scattering to study low-energy spin excitations and their spatial anisotropy in electron-overdoped superconducting BaFe${}_{1.85}$Ni${}_{0.15}$As${}_2$ ($T_c=14$ K). In the normal state, the imaginary part of the dynamic susceptibility, $\ddot{I}{‡}^{\prime \prime }(Q,\ddot{I}‰)$, at the antiferromagnetic (AF) wave vector $Q=(0.5,0.5,1)$ increases linearly with energy for $E\le 13$ meV. Upon entering the superconducting state, a spin gap opens below $E\approx 3$ meV and a broad neutron spin resonance appears at $E\approx 7$ meV. Our careful neutron polarization analysis reveals that $\ddot{I}{‡}^{\prime \prime }(Q,\ddot{I}‰)$ is isotropic for the in-plane and out-of-plane components in both the normal and superconducting states. A comparison of these results with those of undoped BaFe${}_2$As${}_2$ and optimally electron-doped BaFe${}_{1.9}$Ni${}_{0.1}$As${}_2$ ($T_c=20$ K) suggests that the spin anisotropy observed in BaFe${}_{1.9}$Ni${}_{0.1}$As${}_2$ is likely due to its proximity to the undoped BaFe${}_2$As${}_2$. Therefore, the neutron spin resonance is isotropic in the overdoped regime, consistent with a singlet to triplet excitation.