The Study on the Interplay of Superconductivity and Magnetism in Iron based Superconductors

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Addis Ababa University


The study on the interplay of Superconductivity(SC) and Magnetism has attracted much interest on iron based superconductors following the report on LaFeAsO0:89F0:11 to unravel the basic mechanism in high temperature superconductors( HTC). In this dissertation we study the properties of the so called 1111-family of iron based superconductors applying different techniques including, Greens function formalism, magnetization measurement using VSMSQUID, and DFT as implemented in QE and Fleur code. The first part of this work (chapter 3) consists of theoretical calculations applying double time temperature dependent Green function techniques. Starting with a model Hamiltonian consisting of a pairing interaction, magnetic interactions of Heisenberg type by local electrons and an interaction of itinerant electrons with localized electrons, we determined the expression for the superconducting transition temperature, TC. The expressions indicate that magnetization suppresses Superconductivity, and there might be a coexistence below critical temperature. The result is in broad agreement with experimental findings. In the second part (chapter 4), a magnetization measurement with VSM SQUID on three site doped, La1􀀀xYxFe1􀀀yMnyAsO0:89F0:11, samples was done, where the effects of Mn doping on Fe site and Y doping on La site in LaFeAsO1􀀀xFx simultaneously studied, and hence the superconducting critical temperature Tc of different samples was determined. Our results indicate that Y doping has a stabilizing effect on Superconductivity even in the presence of Mn doping. Mn has a pair breaking effect even a small amount of it suppresses TC. Finally, computational work applying the DFT in first principle method as implemented in QE and Fleur code is done(chapter 5). Since iron based superconductors have multi-band character in their electronic structure, we were interested to see the effect of substitution in the band structure and Fermi surface and in turn the effect on TC. The result on the band structures show that Ru substitution changes the hole bands and not the electron bands. One of the possible reason is perhaps that Ru substitution does not induce additional electrons, which is in agreement with NMR and resistivity measurement report. Fermi surfaces (FS) of SmFeAsO show that at Ru = 0.25 it has x a suppressing effect and at Ru = 0.50 a favoring effect on superconductivity. It is predicted that higher TC can be achieved at Ru = 0.50 substitution for SmFeAsO0:85F0:15, in case nesting is an indication for Superconductivity. Calculations indicates that superconductivity can be achieved only for a strong paring potential, which can not be explained by BCS type pairing. Generally, Ru is understood to have effects on LaFeAsO and SmFeAsO compounds and one can not treat these systems with same existing theories keywords: Superconductivity (SC), Iron based Superconductors(IBS), Green Functions, Superconducting quantum interference device (SQUID), Quantum Espresso (QE), Density of state(DOS), Fermi surface (FS), Density functional theory (DFT)



Superconductivity (SC), Iron based Superconductors(IBS), Green Functions, Superconducting quantum interference device (SQUID), Density of state(DOS), Density functional theory (DFT)