Spin gap and magnetic resonance in superconducting BaFe$_{1.9}$Ni$%_{0.1}$As$_{2}$

TL;DR

This study uses neutron spectroscopy to investigate magnetic excitations in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$, revealing a spin gap and resonance linked to electron pairing, with implications for understanding superconductivity mechanisms.

Contribution

It provides detailed insights into the temperature and wave vector dependence of the spin gap and magnetic resonance in this superconductor, highlighting their relation to electron pairing.

Findings

Spin gap opens below $T_c$ and tracks the superconducting gap.

Magnetic resonance energy is temperature and wave vector dependent.

The ratio of spin gap to resonance energy remains constant.

Abstract

We use neutron spectroscopy to determine the nature of the magnetic excitations in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ ($T_{c}=20$ K). Above $T_{c}$ the excitations are gapless and centered at the commensurate antiferromagnetic wave vector of the parent compound, while the intensity exhibits a sinusoidal modulation along the c-axis. As the superconducting state is entered a spin gap gradually opens, whose magnitude tracks the $T$-dependence of the superconducting gap observed by angle resolved photoemission. Both the spin gap and magnetic resonance energies are temperature \textit{and} wave vector dependent, but their ratio is the same within uncertainties. These results suggest that the spin resonance is a singlet-triplet excitation related to electron pairing and superconductivity.