Charge Transport in Nanostructures: Current-Induced Forces and Electronic Friction

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Electrical transport in nanostructures such as atomic wires, molecular junctions or graphene nanoribbons is often accompanied by strong coupling of the electrons to the mechanical degrees of freedom such as local vibrational modes. For higher bias voltages, i.e., far from thermal equilibrium, this coupling gives rise to non-conservative current-induced forces and unusual dissipative phenomena related to electronic friction, which have important consequences. For example, non-conservative current-induced forces can cause strong excitation of local vibrational modes and electronic friction, which the vibrational degrees of freedom experience due to coupling to the electrons, can assume negative values. These effects may cause mechanical instabilities of the nanostructures. A detailed analysis and understanding of the underlying mechanisms is crucial for the further advancement of nanoelectronic architectures. Towards this goal, the current project, which is part of the Research Unit "Reducing complexity of nonequilibrium Systems", involves the development of consistent and accurate theoretical methods to study current-induced mechanical motion of nanosystems in nonequilibrium. To this end, two strategies will be pursued, which both employ the concept of a reduced description of the overall complex problem: (i) the hierarchical quantum master equation approach, a numerically exact method for open quantum systems, and (ii) quantum-classical concepts employing generalized Langevin equations. Applying these two complementary approaches to atomic nanowires between metal electrodes, the signatures, mechanisms and consequences of current induced forces and electronic friction will be analyzed. Furthermore, the emergence of non-conservative forces will be investigated within the semiclassical limit.