Ab initio study of charge transport through single oxygen molecules in atomic aluminum contacts

We present ab initio calculations of transport properties of atomic-sized aluminum contacts in the presence of oxygen. The experimental situation is modeled by considering a single oxygen atom (O) or one of the O-2 and O-3 molecules bridging the gap between electrodes forming ideal, atomically sharp pyramids. The transport characteristics are computed for these geometries with increasing distances between the leads, simulating the opening of a break junction. To facilitate comparison with experiments further, the vibrational modes of the oxygen connected to the electrodes are studied. It is found that in the contact regime, the change of transport properties due to the presence of oxygen is strong and should be detectable in experiments. All three types of oxygen exhibit a comparable behavior in their vibrational frequencies and conductances, which are well below the conductance of pure aluminum atomic contacts. The conductance decreases for an increasing number of oxygen atoms. In the tunneling regime, the conductance decays exponentially with distance and the decay length depends on whether or not oxygen is present in the junction. This fact may provide a way to identify the presence of a gas molecule in metallic atomic contacts.