Level crossing conductance spectroscopy of molecular bridges

A possible mechanism for negative differential resistance is discussed. The level crossing is induced by the source-drain voltage applied across the bridge. The effect is most dramatic when the zero field levels that are resonant with the electrodes are almost degenerate. It is suggested that such degeneracies can arise often when the junctions on either side are weakly coupled by the bridge. Quantitative results for I-V curves are reported on the basis of high-level electronic structure computations for the junction-bridge-junction region and where the electric field is included in the Hamiltonian. (C) 2004 American Institute of Physics.