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| Dissertation / PhD Thesis/Book | PreJuSER-37402 |
2003
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/195
Report No.: Juel-4090
Abstract: The theoretical description of electron transport properties through nanoscale systems is one of the major challenges of contemporary solid state physics. Magnetoelectronics, spin-electronics or molecular-electronics are emerging new fields with a great potentiel for future nano-electronics. In tliis thesis a new method is established, based on the combination of the embedding Green-function method and the full-potential linearized augmented plane-wave (FLAPW) method, to describe the coherent and the sequential electron transport through realistic systems. By use of the density functional theory (DFT) material depended properties of realistic systems can be described on the atomic scale. The FLAPW method is today one of the most reliable and precise numerical methods available for first-principle electronic structure calculations based on the DFT. However, different to standard problems of a periodic solid, the description of electron transport requires the treatment of the scattering problem. This is particularly difficult when described with a plane-wave basis. Thus, a key part of the present thesis is devoted to the development of a new computational scheme which is able to deal with a scattering region sandwiched between two semi-infinite leads. Realizing the ideas put forward by J. Inglesfield, the existing FLEUR code is modified to calculate the single-electron Green function for the embedded scattering region. The semiinfinite leads are described in terms of a transfer-matrix formalism which enables one to obtain the so-called complex bandstructure of bulk materials. The electron transport is described using either the Landauer model or Bardeen's formalism of tunneling. These two formulas are discussed as two different limits of single-particle transport and their reformulation in terms of quantities readily available from the embedding method is presented. Besides the presentation of the theory and the details of its implementation the thesis describes how the method lias been applied to several different test systems to validate the implementation. The spin-dependent transport properties of a Fe/MgO/Fe tunneljunction, the model system of tunnel-magnetoresistance (TMR), were investigated. It is shown that the details of the Fe/MgO interface in this junction is of crucial importance for the tunneling conductance. While the pure relaxation of a Fe/MgO interface already changes the conductance, even more drastic modifications are found as soon as one FeO layer is inserted or if the interface is modified by interchanging the Mg and O atoms.
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