An embedding green function approach for electron transport through interfaces

摘要

In this 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. By the use of the density functional theory realistic systems can be described on the atomic scale. The chosen numerical scheme, the FLAPW method, is today the most reliable and exact method available for first principle electronic structure calculations. However, different to standard bulk setups, the description of electron transport requires the treatment of the scattering problem which is particularly tricky when applying this method. Thus, a key part of the present thesis describes the development of a new computational scheme which is able to deal with a scattering region sandwiched between semi-infinite leads. Based on 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 semi-infinite 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. The method was applied to a multilayer Fe/MgO/Fe setup, the model system of tunnel-magnetoresistance (TMR). 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.