  Negative differential resistance - a decrease in current with increasing bias
voltage - is a counter-intuitive effect that is observed in various molecular
junctions. Here, we present a novel mechanism that may be responsible for such
an effect, based on strong Coulomb interaction between electrons in the
molecule and electrons on the atoms closest to the molecule. The Coulomb
interaction induces electron-hole binding across the molecule-electrode
interface, resulting in a renormalized and enhanced molecule-electrode
coupling. Using a self-consistent non-equilibrium Green's function approach, we
show that the effective coupling is non-monotonic in bias voltage, leading to
negative differential resistance. The model is in accord with recent
experimental observations that showed a correlation between the negative
differential resistance and the coupling strength. We provide detailed
suggestions for experimental tests which may help to shed light on the origin
of the negative differential resistance. Finally, we demonstrate that the
interface Coulomb interaction affects not only the I-V curves but also the
thermoelectric properties of molecular junctions.
