Over the last decade it has become possible to contact single molecules by metallic electrodes and measure the IV characteristic of the resulting nano junction [1,2]. Experiments of this kind can be seen as the first step towards the realization of a molecule based electronics[3]. On a more fundamental level, the IV characteristics provide a spectroscopic fingerprint of the molecular junction containing information about the positions and lifetimes of the electronic energy levels. In view of this, the interpretation of IV curves in terms of the electronic structure of the junction represents a fundamental challenge for molecular electronics.
The standard approach to electron transport in molecular junctions is based on the single-particle Kohn-Sham (KS) scheme of Density Functional Theory (DFT) [4,5]. This description relies on the assumption that the KS eigenvalues represent physically meaningful excitation energies. This is in general not true and therefore the approach has only been partially successful [6,7].
In this work we present a qualitative study of the role of exchange-correlation effects beyond the single-particle picture in electron transport through molecular junctions. We use self-consistent, nonequilibrium many-body perturbation theory (so-called GW method) to calculate the IV curve of a generic two level transport model. Our results demonstrate how the variation of the molecule's energy levels with the bias voltage can produce anomalous features in the dI/dV curve. This effect is suppressed by electronic self-interactions and is therefore underestimated in standard DFT based transport calculations. Inclusion of dynamic correlations introduces quasi-particle scattering which in turn broadens the molecular resonances. The broadening increases strongly with bias and can have a large impact on the calculated IV characteristic. Overall, the study demonstrates that correlation effects can be increasingly important out of equilibrium and indicates that care should be taken in applying DFT to nonequilibrium transport.
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Figure Variation of the HOMO and LUMO levels of the two-level junction as a function of the bias voltage (red lines). Exchange
and correlation has been treated at three different levels, namely Hartree (crosses), Hartree-Fock (triangles), and GW (circles). Notice the qualitatively different ways in which the levels enter the bias window: while the Hartree gap expands, the HF and GW gaps decrease. This is due to the self-interaction in the Hartree potential. While the width of the HOMO and LUMO resonances is independent of bias for Hartree and HF, the width of the GW resonances (indicated by horizontal lines) increases significantly with bias due to enhanced quasiparticle scattering. |
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