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Vibrational properties of iron-chalcogenide superconductor K$_{0.75}$Fe$_{1.75}$Se$_{2}$ with $T_{c}sim$ 30 K have been measured by Raman and optical spectroscopies over temperature range of 3-300 K. Sample undergoes textit{I4/m} $to $ textit{I4} structural phase transition accompanied by loss of inversion symmetry at $T_{1}$, below 250 K, observed as appearance of new fully-symmetric Raman mode at $sim$ 165 cm$^{-1}$. Small vibration mode anomalies are also observed at $T_{2}sim$ 160 K. From first-principles vibrational analysis of antiferromagnetic K$_{0.8}$Fe$_{1.6}$Se$_{2}$ utilizing pseudopotentials all observed Raman and infrared modes have been assigned and the displacement patterns of the new Raman mode identified as involving predominantly the Se atoms.
We report an in-plane optical spectroscopy study on the iron-selenide superconductor K$_{0.75}$Fe$_{1.75}$Se$_2$. The measurement revealed the development of a sharp reflectance edge below T$_c$ at frequency much smaller than the superconducting ener
The electronic structure of the vacancy-ordered K$_{0.5}$Fe$_{1.75}$Se$_2$ iron-selenide compound (278 phase) is studied using the first-principles density functional method. The ground state of the 278 phase is stripe-like antiferromagnetic, and its
We report the results of an experimental study of dc and low frequencies magnetic properties of K$_{0.8}$Fe$_{2}$Se$_2$ single crystal when the dc magnetic field is applied parallel to the $bf{ab}$ plane. From the data obtained, we deduce the full H-
Recent discovery of superconducting (SC) ternary iron selenides has block antiferromagentic (AFM) long range order. Many experiments show possible mesoscopic phase separation of the superconductivity and antiferromagnetism, while the neutron experime
Structural phase separation in A$_x$Fe$_{2-y}$Se$_2$ system has been studied by different experimental techniques, however, it should be important to know how the electronic uniformity is influenced, on which length scale the electronic phases coexis