14.01.25 - Samuel Rudge "Nonadiabatic Dynamics of Molecules Interacting with Metal Surfaces: Electronic Friction and Langevin Dynamics"

When molecules move near a metal surface, nonadiabatic effects due to electronic excitations are unavoidable, which prevents a simple description with adiabatic potential energy surfaces. One popular way to describe such nonadiabatic transitions is electronic friction, which provides a damping mechanism for vibrational motion via electron-hole pair creation. Recently, the idea of electronic friction has permeated the field of molecular electronics. In these setups, a molecule is contacted to two metal electrodes and driven out of equilibrium with a finite voltage bias. In this context, electronic friction arises as a current-induced force acting on the relevant vibrational modes of the molecule, and it connects directly to junction stability, as the positive-definiteness and symmetry of the friction tensor are not guaranteed out of equilibrium. We have developed a method for calculating electronic friction and other current-induced forces based on the numerically exact hierarchical equations of motion (HEOM) approach. Given that HEOM can be applied to a wide variety of systems and in a broad parameter regime, this approach to electronic friction and the resulting mixed quantum-classical equations of motion represents one of the most general approaches to current-induced forces in molecular junctions to date, with applications to several problems in the context of transport in nanostructures, such as waterwheel-type nuclear motion, interacting systems, and current-induced bond rupture.