Effect of iron content and potassium substitution in ${A}_{0.8}$Fe${}_{1.6}$Se${}_{2}$ ($A$=K, Rb, Tl) superconductors: A Raman scattering investigation

Effect of iron content and potassium substitution in $A_{0.8}$ Fe $_{1.6}$ Se $_{2}$ ( $A$ =K, Rb, Tl) superconductors: A Raman scattering investigation

A. M. Zhang, K. Liu, J. B. He, D. M. Wang, G. F. Chen, B. Normand, and Q. M. Zhang

Phys. Rev. B 86, 134502 – Published 1 October 2012

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 $A$ FeSe 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 60 cm $^{- 1}$ , 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.