Effect of iron content and potassium substitution inA0.8Fe1.6Se2

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 non-superconducting 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.

Improved critical current densities in bulk FeSe superconductor using ball milled powders and high temperature sintering

“…FeSe has a tetragonal PbO structure with space group of P4/nmm. From the symmetry analysis, one can find that the Raman active phonon modes originating from the zone center are . The phase formation in the present sample is confirmed by the presence of Raman modes at 180 cm −1 and denoted by F1, which is due to the stretching modes of Se atoms and at 220 cm −1 denoted by F2, is due to the stretching vibration of the Fe atoms within the unit cell of FeSe.…”

Superconductivity in Fe-chalcogenides

“…al. performed systematic Raman studies on K 0.8 Fe 1.6 Se 2 , Tl 0.5 K 0.3 Fe 1.6 Se 2 , and Tl 0.5 Rb 0.3 Fe 1.6 Se 2 , together with first-principles calculations [81], showing that the abundant phonon modes in A x Fe 2-y Se 2 are the consequence of the superstructure with ordered iron vacancies, which can be assigned by the calculation reasonably well. In the high-energy region, the Raman spectra of A x Fe 2-y Se 2 exhibit a broad, asymmetric peak around 1600 cm -1 , which was identified as a two-magnon process involving optical magnons [82].…”

Phase separation, competition, and volume-fraction control inNaFe1−xCoxAs

“…The concept of "nanoscale phase separation" in ironbased superconductors is known from the depleted iron selenide materials A 2 Fe 4 Se 5 (A = K, Rb, Cs, Tl; "245") 18 , which appear to show a robust AFM phase accompanied by an equally robust but quite separate PM phase; the latter is the only part of the system to turn SC at T c , forming a percolating SC phase despite hav-ing a volume fraction below 10%. However, it is generally thought that this phase separation is primarily a consequence of vacancy-induced structural inhomogeneity, causing a clear doping inhomogeneity, whereas our results (previous paragraph) appear to exclude this in NaFe 1−x Co x As.…”