Vacancy ordering and phonon spectrum of the iron-based superconductor K <inf>0.8</inf>Fe <inf>1.6</inf>Se <inf>2</inf>

Abstract

We have performed Raman-scattering measurements on a high-quality single crystal of the recently discovered Fe-based superconductor K 0.8 Fe 1.6 Se 2 (T c = 32 K). At least 13 phonon modes were observed in the wave number range 10-300 cm - 1. The spectra possess a fourfold symmetry indicative of bulk vacancy order in the Fe-deficient planes. We perform a vibration analysis based on first-principles calculations, which both confirms the ordered structure and allows a complete mode assignment. We observe an anomaly at T c in the 180 cm - 1 A g mode, which indicates a rather specific type of electron-phonon coupling. © 2012 American Physical Society.