| There are two main difficulties for the first principles study of transport properties at the nano scale. The first is that many-body interactions need to be taken into account for the infinite system without periodic boundary conditions. The other is that the system is usually in a non-equilibrium state. Both of these two difficulties are beyond the ability of conventional first principles methods to reconcile. Recently, a new first principles approach which combines the Non-equilibrium Green's Functions Technique (NGFT) and the Density Functional Theory (DFT) was proposed. DFT has been proved to be successful in molecular and solid state physics. Currently used DFT approximations can take into account 'most' many-body effects and NGFT naturally includes the non-equilibrium effects. The new approach uses NGFT to treat the non-periodic boundary conditions and DFT to treat many body interactions. This approach has been successfully used in molecular electronics. The thesis is organized in the following way. First: we introduce the main ideas of combining NGFT and DFT, and then apply this method to a light-driven molecular switch. The switch, made of a single molecule, is one of the most important elements of nano-electronics. However, most proposed molecular switches are driven either by an external bias voltage or by STM manipulation, neither of which is ideal for nano-scale circuits. The switch we designed has a high on-off conductance ratio and more importantly, can be driven by photons. In following chapters, we generalize the method to the spin-dependent case and apply it to a magnetic layered structure. We implemented the method within the framework of the Layer Korringa-Kohn-Rostoker (LKKR) approach, which is particularly well-adapted to the layered structure and found a bias-enhanced tunneling magneto-resistance (TMR) for the Fe/FeO/MgO/Fe junction. Our results are important not only for application, but also for understanding of the voltage-dependence of TMR for layered structures. The experimental studies show that the bias voltage usually kills the TMR of amorphous magnetic tunneling junctions. Our study shows that for an impurity-free layered structure, a different behavior of TMR may occur. |
