Browsing by Author "Mebrahtu, Abera"

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    Electric Field and Pressure Enhanced Magnetic Properties of Multiferroic Materials and Bifeo3, and Enhancement of Critical Temperature of Superconductor(Smasfeof) Under Pressure
    (Addis Ababa University, 2018-03-13) Mebrahtu, Abera; Singh, Pooran (Professer)
    In this dissertation we have studied electric field enhanced magnetic properties of multiferroic materials . The system contains a ferromagnetic and a ferroelectric sub lattices. To describe the property of system, we considered linear couplingHamiltonian between ferromagnetism and ferroelectricity without microscopic derivation. The thermodynamic properties of the system are calculated, such as magnon excitation spectrum , temperature dependences of magnetization, internal energy, specific heat capacity, and magnetic susceptibility subjected to electric and magnetic fields. In addition to this electronic and structural properties of multiferroic material(BiFeO3) are studied using density functional theory. We also investigate critical temperature of iron based superconductors (SmAsFeOF) under the influence of an external hydrostatic pressure based on density functional theory calculations using Vasp and quantum Espresso Code coupled with Migdal Eliashberg model. Our results show that the critical temperature increases up to 45.91K under the pressure upto 30GPa and, subsequently, drops down until 60GPa. Throughout this pressure range, the system is preserving the initial symmetry without any structural transformation. Our results indicate that the enhancement of critical temperature is due to increase electron boson (phonon/spin fluctuation) coupling and further increase of pressure may lead to instability of system. Our theoretical results are in agreement with the experiments.
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    Study of coexistence of Superconductivity and Antiferromagnetism in smaso1􀀀xfxfe
    (Addis Ababa University, 2011-06) Mebrahtu, Abera; P. Singh (Professor)
    Superconductivity and magnetism were previously thought as incompatible until the discovery of some rare earth ternary compounds that show the coexistence of superconductivity and magnetism. In some of the recently discovered iron-based layered superconductors, superconductivity and magnetism coexist. In the present work we examine the possibility of coexistence of antiferromagnetism and superconductivity in samarium arsenide oxide superconductor (SmAsO1􀀀xFxFe). Using a model of the Hamiltonian and retarded double time Greens function formalism, we found expressions for superconducting transition temperature (Tc), superconductivity order parameter ( sc(T)), AFM order parameter ( ), and AFM transition temperature (Tm). We obtained the phase diagrams (Tcvs ) and (Tmvs ) to obtain the region where both orders ,i.e, superconductivity and AFM coexist. The region under the intersection of the two merged graphs shows that superconductivity and AFM coexist in the system(SmAsO1􀀀xFxFe)