  The interaction between electronic and vibrational degrees of freedom is an
important mechanism in nonequilibrium charge transport through molecular
nanojunctions. While adiabatic polaron-type coupling has been studied in great
detail, new transport phenomena arise for nonadiabatic coupling scenarios
corresponding to a breakdown of the Born-Oppenheimer approximation. Employing
the numerically exact hierarchical equations of motion approach, we analyze the
effect of nonadiabatic electronic-vibrational coupling on electron transport in
molecular junctions considering a series of models with increasing complexity.
The results reveal a significant influence of nonadiabatic coupling on the
transport characteristics and a variety of interesting effects, including
negative differential conductance. The underlying mechanisms are analyzed in
detail.
