Superconductivity-induced optical anomaly in an iron arsenide

TL;DR

This study reveals that superconductivity in iron arsenides affects optical properties at energies far above the gap, indicating deep electronic states contribute to superconductivity, challenging conventional theories.

Contribution

It provides the first spectroscopic evidence that high-energy electronic states are involved in superconductivity in iron arsenides, contrary to traditional models.

Findings

Superconductivity suppresses an optical absorption band at 2.5 eV.

Deep As-p orbitals influence superconducting and magnetic states.

High-energy electronic transitions are affected by superconductivity.

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 $\textrm{Ba}_{0.68}\textrm{K}_{0.32}\textrm{Fe}_2\textrm{As}_2$ that challenges this notion. We observe a superconductivity-induced suppression of an absorption band at an energy of $2.5\ \textrm{eV}$, two orders of magnitude above the superconducting gap energy $2\Delta\sim 20\ \textrm{meV}$. Based on density-functional calculations, this band can be assigned to transitions from As-p to Fe-d orbitals crossing the Fermi surface. 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.