Dual Character of Magnetism in Ferropnictides: Insights from Optical Measurements

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We investigate the electronic structure of EuFe₂As₂ using optical spectroscopy and first-principles calculations. At low temperature we observe the evolution of 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 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.