Browsing by Author "Mekonnen, Sintayehu"

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    First Principle Investigation of Electronic Structure,Magnetic Property and Valley Hall Conductivity Inmonolayer and Bilayermos2 Doped (Mn,V)
    (Addis Ababa University, 2018-02-02) Mekonnen, Sintayehu; Singh, Pooran (Professer)
    In the first and second parts of this dissertation structural stability, electronic structure and magnetic interaction in transition metal(TM) atoms(V,Mn) doped monolayer (ML) and bilayer(BL)MoS2 are studied within the density function theory(DFT) based on DFT+U formalism. It is found that,the injection of V andMn atoms inMo site of ML and BL MoS2 introduces magnetism and turns semiconductor behavior of host MoS2 to half metallic nature. The magnetic interaction between dopants in ML and BL MoS2 are always ferromagnetic irrespective of dopant configurations. In contrast, in V doped case the magnetic interaction oscillates from ferromagnet to antiferromagnet depending on the separation between dopants. Moreover,it is found that interlayer interaction in doped BL MoS2 system affects not only electronic structures but also the magnetic properties of the system. The calculated ferromagnetic transition temperature (TC) inMn doped ML and BLMoS2 cases are found to be above the room temperature (RT), whereas in V doped cases TC closer to RT. In addition, TC increases with doping concentrations in a range of dilute limit (6.5%) of magnetic atoms for doped ML and BL MoS2,which agrees with latest experimental observed RT TC .Therefore based on our result, we suggest thatMn and V doped ML and BL MoS2 are promising candidates for 2D DMS for high temperature spintronics applications. In third part of this dissertation,using low energy effective tight binding model together with consideration of dopant introduced exchange field it is found that spin orbit coupling together with exchange energy determine the valley polarization which in turn controls valley and spin Hall conductivity in doped ML MoS2 system. Our results form another important step towards information processing based on the valley degree of freedom.
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    Superconductivity and Jahn Teller Effect in LA(O1􀀀XFX)Feas Superconductor
    (Addis Ababa University, 2010-07) Mekonnen, Sintayehu
    The recent discovery of superconductivity at moderately high temperature(26K to 55K) in doped iron-based pnictides(RnO1􀀀xFxFeAs),where Rn=La,Ce,Sm,Pr,Nd.etc) having layered-structure like cuprates,has triggered challenge towards understanding the paring mechanism. In this work we analyze superconductivity and the Jahn Teller e ect in this newly discovered La(O1􀀀xFx)FeAs superconductor. after reviewing the current nding on this systems, we suggested in phonon-mediated and distortion- eld-mediated paring processes give the right order of superconducting critical temperature Tc.The distortion- eld modes arise from Jahn-Teller e ects due to degenerate or near-degenerate iron 3dxz and 3dyz orbitals. The interaction of these degenerate electrons with phonon and distortion eld results change of shape of orbitals consequentially a change in splitting of electronic orbitals. Ac- cording to our calculated result in chapter three that superconducting parameters;energy gap ( ) no of particles inside Fermi surface(N(0)),coe cient of isotopic constant ( ) are a ected due to Jahn-Teller distortion,consequently superconducting transition tempera- ture (Tc) is a ected . The calculated parameters are coincide with experimental values for suitable values of constants