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We have performed Raman-scattering measurements on a high-quality single crystal of the recently discovered Fe-based superconductor K0.8Fe1.6Se2 (Tc = 32 K). At least thirteen phonon modes were observed in the wave number range 10−300 cm −1 . The spectra possess a four-fold 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 Tc in the 180 cm −1 Ag mode, which indicates a rather specific type of electron-phonon coupling.

The antiferromagnetic (AFM) insulator-superconductor transition has been always a center of interest in the underlying physics of unconventional superconductors. The quantum phase transition between Mott insulator with antiferromagnetism and superconductor can be induced by doping charge carriers in high-T c cuprate superconductors 1 . For the best characterized organic superconductors of κ-(BEDT-TTF) 2 X (X=anion), a first order transition between antiferromagnetic insulator and superconductor can be tuned by applied external pressure or chemical pressure 2-4 . Also, the superconducting state can be directly developed from antiferromagnetic insulator by application of pressure in Cs 3 C 60 5 . The resemblance of these phase diagrams hints a universal mechanism governing the unconventional superconductivity in close proximity to antiferromagnetic insulators. However, the superconductivity in iron-based high-T c superconductors evolves from an antiferromagnetic bad metal by doping charge carriers, and no superconductor-insulator transition has been observed so far 6-8 . Here, we report a first-order transition from superconductor to insulator with a strong charge doping induced by ionic gating in the thin flakes of single crystal (Li,Fe)OHFeSe. The superconducting transition temperature (T c ) is

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.

We performed Raman-scattering measurements on high-quality single crystals of A 0.8 Fe 1.6 Se 2 superconductors of several compositions. We found a broad, asymmetric peak around 1600 cm −1 (200 meV), which we identify as a two-magnon process involving optical magnons. The intensity of the two-magnon peak falls sharply on entering the superconducting phase. This effect, which is entirely absent in the non-superconducting system KFe 1.5 Se 2 , requires a strong mutual exclusion between antiferromagnetism and superconductivity arising from proximity effects within regions of microscale phase separation.