Vacancy ordering and phonon spectrum of the iron-based superconductor K0.8Fe1.6Se2

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${}^{\ensuremath{-}1}$. 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${}^{\ensuremath{-}1}$ ${A}_{g}$ mode, which indicates a rather specific type of electron-phonon coupling.