Length and temperature dependent crossover of charge transport across molecular junctions

We study the electronic transport in a molecular junction in which each site is coupled to a local phonon bath using the non-equilibrium Green's function method. We observe the length period of the oscillatory conductance in odd-numbered chains depends strongly on the applied bias, and the oscillatory behavior is smeared out for the bias voltage near the phonon energy. In addition, a crossover from tunneling to thermally activated hopping transport as the length of the molecule increases is found for the phonon-free case. In the presence of electron-phonon interaction, hopping transport is dominant and a transition from the thermally suppressed to assisted conduction is observed.