Anisotropic Neutron Spin Resonance in Superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_2$

TL;DR

This study uses polarized neutron scattering to demonstrate that the neutron spin resonance in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_2$ is purely magnetic and anisotropic, involving only specific triplet components, challenging previous isotropic assumptions.

Contribution

It reveals the anisotropic nature of the neutron spin resonance and identifies the specific triplet components involved in the superconducting state.

Findings

Resonance is purely magnetic in origin.

Resonance occurs only for the planar response.

Only the $S_{001}= ext{±}1$ triplet components are involved.

Abstract

We use polarized inelastic neutron scattering to show that the neutron spin resonance below $T_c$ in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_2$ ($T_c=20$ K) is purely magnetic in origin. Our analysis further reveals that the resonance peak near 7~meV only occurs for the planar response. This challenges the common perception that the spin resonance in the pnictides is an isotropic triplet excited state of the singlet Cooper pairs, as our results imply that only the $S_{001}=\pm1$ components of the triplet are involved.