Electron-doping evolution of the low-energy spin excitations in the iron arsenide BaFe$_{2-x}$Ni$_{x}$As$_{2}$ superconductors

TL;DR

This study uses neutron scattering to explore how low-energy spin excitations in BaFe2-xNixAs2 evolve with electron doping, revealing a correlation between spin resonance features and superconductivity across different doping levels.

Contribution

It provides a systematic analysis of spin excitation evolution with doping, highlighting the persistence of three-dimensional resonance features in superconducting phases.

Findings

Superconductivity appears with a neutron spin resonance and reduced static magnetic order.

The spin resonance energy correlates with doping and persists into the overdoped regime.

Three-dimensional spin excitations are prevalent throughout the phase diagram.

Abstract

We use elastic and inelastic neutron scattering to systematically investigate the evolution of the low-energy spin excitations of the iron arsenide superconductor BaFe2-xNixAs2 as a function of nickel doping x. In the undoped state, BaFe2As2 exhibits a tetragonal-to-orthorhombic structural phase transition and simultaneously develops a collinear antiferromagnetic (AF) order below TN = 143 K. Upon electron-doping of x = 0.075 to induce bulk superconductivity with Tc = 12.3 K, the AF ordering temperature reduces to TN = 58 K.We show that the appearance of bulk superconductivity in BaFe1.925Ni0.075As2 coincides with a dispersive neutron spin resonance in the spin excitation spectra, and a reduction in the static ordered moment. For optimally doped BaFe1.9Ni0.1As2 (Tc = 20 K) and overdoped BaFe1.85Ni0.15As2 (Tc = 15 K) superconductors, the static AF long-range order is completely suppressed and the spin excitation spectra are dominated by a resonance and spin-gap at lower energies. We determine the electron-doping dependence of the neutron spin resonance and spin gap energies, and demonstrate that the three-dimensional nature of the resonance survives into the overdoped regime. If spin excitations are important for superconductivity, these results would suggest that the three-dimensional character of the electronic superconducting gaps are prevalent throughout the phase diagram, and may be critical for superconductivity in these materials.