Dual character of magnetism in EuFe₂As₂: Optical spectroscopic and density-functional calculation study

We investigate the electronic structure of EuFe₂As₂ using optical spectroscopy and density-functional calculations. At low temperature we observe the evolution of two gaplike features, one having a weak-coupling mean-field behavior and another with strongly nonmean-field behavior. Using band-structure calculations, we identify the former with a spin-Peierls-type partial gap in dyz bands and the latter with the transition across the large exchange gap in dxz / dxy bands. Our results demonstrate that the antiferromagnetism in the ferropnictides is neither fully local nor fully itinerant but contains elements of both.