Static and Dynamic Magnetism in Underdoped Superconductor BaFe$_{1.92}$Co$_{0.08}$As$_2$

TL;DR

This study uses neutron scattering to explore the interplay of static and dynamic magnetism in underdoped BaFe$_{1.92}$Co$_{0.08}$As$_2$, revealing a spin resonance and competition between magnetic order and superconductivity.

Contribution

It provides detailed measurements of magnetic excitations and their evolution across the superconducting transition in an underdoped iron-based superconductor.

Findings

Magnetic Bragg peak intensity decreases by 6% below T_C

A spin resonance appears at ~4.5 meV below T_C

No additional gapping observed down to 2 meV

Abstract

We report neutron scattering measurements on single crystals of BaFe$_{1.92}$Co$_{0.08}$As$_2$. The magnetic Bragg peak intensity is reduced by 6 % upon cooling through T$_C$. The spin dynamics exhibit a gap of 8 meV with anisotropic three-dimensional (3d) interactions. Below T$_C$ additional intensity appears at an energy of $\sim$4.5(0.5) meV similar to previous observations of a spin resonance in other Fe-based superconductors. No further gapping of the spin excitations is observed below T$_C$ for energies down to 2 meV. These observations suggest the redistribution of spectral weight from the magnetic Bragg position to a spin resonance demonstrating the direct competition between static magnetic order and superconductivity.