Polarized neutron scattering studies of magnetic excitations in electron-overdoped superconducting BaFe$_{1.85}$Ni$_{0.15}$As$_{2}$

TL;DR

This study uses polarized neutron scattering to investigate spin excitations in overdoped BaFe$_{1.85}$Ni$_{0.15}$As$_{2}$, revealing isotropic magnetic responses and insights into the nature of the neutron spin resonance in this superconductor.

Contribution

It provides the first detailed polarized neutron scattering analysis of spin excitations in overdoped BaFe$_{1.85}$Ni$_{0.15}$As$_{2}$, showing isotropic susceptibility and comparing it with other doping levels.

Findings

Spin excitations are isotropic in both normal and superconducting states.

A spin gap opens below 3 meV in the superconducting state.

Neutron spin resonance appears at 7 meV, consistent with singlet to triplet excitation.

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, $\chi^{\prime\prime}(Q,\omega)$, 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 $\chi^{\prime\prime}(Q,\omega)$ 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.