Theoretical studies of quantum transport in molecular junctions using the density matrix hierarchy method: Nonadiabatic effects, anharmonic vibrations, and current fluctuations

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Quantum transport in molecular junctions, that is a single molecule chemically bound to metal or semiconductor electrodes, is an active field of experimental and theoretical research. Molecular junctions provide the possibility to study fundamental aspects of nonequilibrium many-body quantum physics at the nanoscale and have been of great interest in the field of molecular electronics. From the point of view of theory, the quantitative description of transport in molecular junctions represents a significant challenge. Despite the progress in recent years, there is a lack of very accurate transport methods that can be applied to realistic models of molecular junctions. In this project, the density matrix hierarchy method shall be further developed and implemented for a general model of molecular junctions. This will provide a methodology which extends the range of systems addressable by numerically exact methods significantly, including in particular models with nonadiabatic coupling and realistic potential energy surfaces. This is of importance for low-frequency vibrations, where the widely used harmonic approximation is often invalid, and is indispensable for systems, which exhibit large amplitude motion such as, for example, torsional motion or molecular switches based on conformational changes. The methodology will be used to investigate a variety of interesting, but so far largely unexplored mechanisms and phenomena in molecular junctions including nonadiabatic effects induced by conical intersection of potential energy surfaces, anharmonic vibrational motion in systems with large amplitude motion, such as for example torsional motion in oligophenylenes, as well as current noise in these type of systems.