Conductance calculation of hydrogen molecular junctions between Cu electrodes

Transport in H2 molecules sandwiched between Cu electrodes is investigated by first principles. The theoretical results indicate the presence of two regimes characterized by high and low conductance values. Both of them comprise the molecule within the junction but differ in the adsorption geometry. An experimentally observed low conductance state is tentatively attributed to an asymmetric adsorption site at short tip-tip distances, while at longer tip-tip distances the H2 molecule is coaxial and inside the junction, giving a conductance comparable to that of the clean contact. The threshold energy for the breakdown of the ballistic regime in the high conductance configuration is estimated as the lowest vibrational energy of the bridging H2 molecule.