The effect of bonding of a CO molecule on the conductance of atomic metal wires

We have measured the effect of bonding of a CO molecule on the conductance of Au, Cu, Pt and Ni atomic contacts at 4.2 K. When CO gas is admitted to the metal nanocontacts, a conductance feature appears in the conductance histogram near 0.5 of the quantum unit of conductance, for all metals. For Au, the intensity of this fractional conductance feature can be tuned with the bias voltage, and it disappears at high bias voltage (above approximately 200 mV). The bonding of CO to Au appears to be weakest, and associated with the formation of monatomic Au wire.