Dual Character of Magnetism in Ferropnictides: Insights from Optical Measurements

We investigate the electronic structure of EuFe${}_2$As${}_2$ using optical spectroscopy and first-principles calculations. At low temperature we observe the evolution of $\mathit{\text{two}}$ gap-like features, one having a BCS mean-field behavior and another with strongly non-BCS behavior. Using band structure calculations, we identify the former with a spin-Peierls-like partial gap in $d_{yz}$ bands, and the latter with the transition across the large exchange gap in $d_{xz}\mathrm{/}{d}_{xy}$ bands. Our results demonstrate that the antiferromagnetism in the ferropnictides is neither fully local nor fully itinerant, but contains elements of both.