Effect of iron content and potassium substitution in A0.8Fe1.6Se2 (A=K, Rb, Tl) superconductors: A Raman scattering investigation

Abstract

We have performed Raman-scattering measurements on high-quality single crystals of the superconductors K0.8Fe1.6Se2 (Tc=32 K), Tl0.5K0.3Fe1.6Se2 (Tc=29 K), and Tl0.5Rb0.3Fe1.6Se2 (Tc=31 K) as well as of the insulating compound KFe1.5Se2. To interpret our results, we have made first-principles calculations for the phonon modes in the ordered iron-vacancy structure of K0.8Fe1.6Se2. 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 A0.8Fe1.6Se2 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 K0.8Fe1.6Se2 structure.

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