A First-Principles, Tight Binding and Coherent Potential Study of 1-1 Type Iron-based Superconductor FeSe(x)Te(1-x)




Koufos, Alexander P.

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In this dissertation I performed first-principles calculations of the electronic structure for Iron-Selenium-Tellurium (FeSeTe) systems and applied the results to study superconductivity in these materials. This dissertation discusses new first-principles calculations based on the Linearized Augmented Plane Wave (LAPW) method of Density Functional Theory (DFT) used in the Gaspari-Gyorffy-McMillan (GGM) theories of superconductivity. The LAPW results were also used to construct Tight-Binding (TB) Hamiltonians using the Naval Research Laboratory TB (NRL-TB) method. The code was expanded to include angular momentum contribution to the fits, to improve total and decomposed Density of States (DOS). Final fits for FeSe and FeTe showed total energy and total DOS calculations to compare well with the LAPW results up to the eighteenth band. These fits were further used to develop a computer program to treat disorder effects via the Coherent Potential Approximation (CPA) to the TB method. The new code incorporated diagonal disorder in the CPA and combined with the Virtual Crystal Approximation to study various concentrations of FeSe(x)Te(1-x), where random substitutions of Se by Te were taken into account. Calculated DOS results with the CPA show similar features to the calculated LAPW DOS of FeSe(0.50)Te(0.50) by a supercell approach. The results presented show that the GGM theories based on LAPW results can find good agreement with experimentally measured superconductivity temperatures, T(c). Specifically, we calculate T(c)'s of 5.4K and 14.5K for FeSe and FeSe(0.50)Te(0.50), respectively. The corresponding calculated electron-phonon coupling constants, lambda, are 0.64 and 0.97, respectively. This suggests that the electron-phonon coupling interaction of the Bardeen-Cooper-Schrieffer (BCS) theory is an important mechanism in the superconductivity of these materials.



Coherent Potential Approximation, Electronic Structure, Electron-Phonon Coupling, Iron-based Superconductivity, Linearized Augmented Plane Wave Method, Tight-Binding Method