title: Superconductivity-induced optical anomaly in an iron arsenide.
abstract: One of the central tenets of conventional theories of superconductivity, including most models proposed for the recently discovered iron-pnictide superconductors, is the notion that only electronic excitations with energies comparable to the superconducting energy gap are affected by the transition. Here, we report the results of a comprehensive spectroscopic ellipsometry study of a high-quality crystal of superconducting Ba₀.₆₈K₀.₃₂Fe₂As₂ that challenges this notion. We observe a superconductivity-induced suppression of an absorption band at an energy of 2.5 eV, two orders of magnitude above the superconducting gap energy 2Δ≈20 meV. On the basis of density functional calculations, this band can be assigned to transitions from As-p to Fe-d orbitals crossing the Fermi level. We identify a related effect at the spin-density wave transition in parent compounds of the 122 family. This suggests that As-p states deep below the Fermi level contribute to the formation of the superconducting and spin-density wave states in the iron arsenides.
authors.affiliation: Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, Stuttgart D-70569, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Charnukha
authors.forename: P
authors.initials: P
authors.lastname: Popovich
authors.forename: Y
authors.initials: Y
authors.lastname: Matiks
authors.forename: D L
authors.initials: DL
authors.lastname: Sun
authors.forename: C T
authors.initials: CT
authors.lastname: Lin
authors.forename: A N
authors.initials: AN
authors.lastname: Yaresko
authors.forename: B
authors.initials: B
authors.lastname: Keimer
authors.forename: A V
authors.initials: AV
authors.lastname: Boris