Shevtsova T. Lattice dynamics and electronic structure of low-dimensional conducting iron-based systems

The thesis is devoted studying of lattice dynamics and electronic structure of iron superconductors and narrow carbon nanotubes contained iron chains. Self-consistent calculations were carried out within the framework of the theory of functional density using the full potential linear augmented plane wave method and the pseudopotential one to obtain the electronic structure and phonon frequencies of the objects under study. As a result, an impact of the iron magnetic moment and its spin state on the phonon subsystem of the FeTe, FeSe, and Rb2Fe4Se5 iron chalcogenides was investigated. Anomalous softening of high-frequency phonons under increasing magnetic moment of iron as well as high sensitivity of the vibrational B1g(Fe) mode to the change of the iron spin state were found. It was shown that a change in the iron spin state is one of the reasons for the anomalously large hardening of the B1g(Fe) phonon mode in FeSe observed experimentally. For the phase-separated Rb0.8+xFe1.6+ySe2 compound, the phonon frequencies were obtained in the different phases. Ab-initio spin-polarized band structure calculations showed that the compressed vacancy ordered Rb2Fe4Se5 phase can be conductive one and, therefore, it may serve as a protective interface spacer between the purely metallic RbδFe2Se2 phase and the insulating Rb2Fe4Se5 phase providing percolative Josephson-junction like superconductivity all throughout of Rb0.8+xFe1.6+ySe2. The structural stability, as well as the magnetic and conducting properties of the composites designed from carbon nanotubes and iron nanowires, were tested depending on the geometric parameters and different arrangement of iron ions in the nanotubes.