Effect of iron content and potassium substitution in A <inf>0.8</inf>Fe <inf>1.6</inf>Se <inf>2</inf> (A=K, Rb, Tl) superconductors: A Raman scattering investigation

Abstract

We have performed Raman-scattering measurements on high-quality single crystals of the superconductors K 0.8 Fe 1.6 Se 2 (T c =32 K), Tl 0.5 K 0.3 Fe 1.6 Se 2 (T c =29 K), and Tl 0.5 Rb 0.3 Fe 1.6 Se 2 (T c =31 K) as well as of the insulating compound KFe 1.5 Se 2 . To interpret our results, we have made first-principles calculations for the phonon modes in the ordered iron-vacancy structure of K 0.8 Fe 1.6 Se 2 . The modes we observe can be assigned very well from our symmetry analysis and calculations, allowing us to compare Raman-active phonons in the AFeSe compounds. We find a clear frequency difference in most phonon modes between the superconducting and nonsuperconducting potassium crystals, indicating the fundamental influence of iron content. By contrast, substitution of K by Tl or Rb in A 0.8 Fe 1.6 Se 2 causes no substantial frequency shift for any modes above 60cm - 1, demonstrating that the alkali-type metal has little effect on the microstructure of the FeSe layer. Several additional modes appear below 60 cm - 1 in Tl- and Rb-substituted samples, which are vibrations of heavier Tl and Rb ions. Finally, our calculations reveal the presence of "chiral" phonon modes, whose origin lies in the chiral nature of the K 0.8 Fe 1.6 Se 2 structure. © 2012 American Physical Society.