Nuclear coupling and polarization in molecular transport junctions: Beyond tunneling to function

Much current experimental research on transport in molecular junctions focuses on finite voltages, where substantial polarization- induced nonlinearities may result in technologically relevant device- type responses. Because molecules have strong polarization responses to changing charge state or external field, molecules isolated between electrodes can show strongly nonlinear current- voltage responses. For small applied voltages ( up to similar to 0.3 volt), weak interaction between transporting electrons and molecular vibrations provides the basis for inelastic electron tunneling spectroscopy. At higher voltages and for certain time scale regimes, strong coupling effects occur, including Coulomb blockade, negative differential resistance, dynamical switching and switching noise, current hysteresis, heating, and chemical reactions. We discuss a general picture for such phenomena that arise from charging, strong correlation, and polarization ( electronic and vibrational) effects in the molecule and at the interface.