Anisotropic resonance modes emerging in an antiferromagnetic superconductor

The emergence of spin-resonance modes in an antiferromagnetic superconductor was studied on Co underdoped BaFe₂As₂, for which the local coexistence of large moment antiferromagnetism and superconductivity is well established by local probes. Several unpolarized neutron scattering studies for Co doping of about 4.5% reveal a broad spin-resonance feature in the superconducting state but could not identify the nature of this signal. We have applied the polarization analysis in our neutron scattering experiments, which allows us to separate and identify the anisotropies of the magnetic excitations. In the normal state of Co underdoped BaFe₂As₂ the antiferromagnetic order results in broad magnetic gaps opening in all three spin directions that are reminiscent of the magnetic response in the parent compound [1], where magnetic anisotropy arising from spin-orbit coupling results in sizeable magnetic gaps. In the superconducting state of the underdoped material two distinct anisotropic resonance excitations can be identified, but in contrast to numerous studies on optimum and over-doped BaFe₂As₂ there is no isotropic resonance excitation [1], in particular there is no longitudinal contribution to the superconducting resonance.
[1] F. Waßer et al., Sci. Rep. 7, 10307 (2017).