The Study On The Interplay Of Superconductivity And Magnetism In Iron Based Superconductors

The study on the interplay of Superconductivity(SC) and Magnetism has attractedrnmuch interest on iron based superconductors following the report onrnLaFeAsO0:89F0:11 to unravel the basic mechanism in high temperature superconductors(rnHTC). In this dissertation we study the properties of the so calledrn1111-family of iron based superconductors applying different techniques including,rnGreens function formalism, magnetization measurement using VSMSQUID,rnand DFT as implemented in QE and Fleur code.rnThe first part of this work (chapter 3) consists of theoretical calculations applyingrndouble time temperature dependent Green function techniques. Startingrnwith a model Hamiltonian consisting of a pairing interaction, magnetic interactionsrnof Heisenberg type by local electrons and an interaction of itinerantrnelectrons with localized electrons, we determined the expression for the superconductingrntransition temperature, TC. The expressions indicate that magnetizationrnsuppresses Superconductivity, and there might be a coexistence belowrncritical temperature. The result is in broad agreement with experimental findings.rnIn the second part (chapter 4), a magnetization measurement with VSM SQUIDrnon three site doped, La1À€€xYxFe1ð€€€yMnyAsO0:89F0:11, samples was done, where therneffects of Mn doping on Fe site and Y doping on La site in LaFeAsO1ð€€€xFx simultaneouslyrnstudied, and hence the superconducting critical temperature Tcrnof different samples was determined. Our results indicate that Y doping has arnstabilizing effect on Superconductivity even in the presence of Mn doping. Mnrnhas a pair breaking effect even a small amount of it suppresses TC.rnFinally, computational work applying the DFT in first principle method as implementedrnin QE and Fleur code is done(chapter 5). Since iron based superconductorsrnhave multi-band character in their electronic structure, we wererninterested to see the effect of substitution in the band structure and Fermirnsurface and in turn the effect on TC. The result on the band structures showrnthat Ru substitution changes the hole bands and not the electron bands. Onernof the possible reason is perhaps that Ru substitution does not induce additionalrnelectrons, which is in agreement with NMR and resistivity measurementrnreport. Fermi surfaces (FS) of SmFeAsO show that at Ru = 0.25 it hasrnxrna suppressing effect and at Ru = 0.50 a favoring effect on superconductivity.rnIt is predicted that higher TC can be achieved at Ru = 0.50 substitution forrnSmFeAsO0:85F0:15, in case nesting is an indication for Superconductivity. Calculationsrnindicates that superconductivity can be achieved only for a strongrnparing potential, which can not be explained by BCS type pairing. Generally,rnRu is understood to have effects on LaFeAsO and SmFeAsO compounds andrnone can not treat these systems with same existing theoriesrnkeywords: Superconductivity (SC), Iron based Superconductors(IBS), Green Functions,rnSuperconducting quantum interference device (SQUID), Quantum Espressorn(QE), Density of state(DOS), Fermi surface (FS), Density functional theory (DFT)

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