A mechanical perturbation approach to study quantized conductance in atomic point contacts

Atomic point contacts down to a single atom have been studied using gold as a model system. Having only a single valence electron, the conductance through one gold atom is predicted to be one quantum unit of conductance (the quantum unit of conductance being 1 Go= 2e2/h =7.748091 x 10-5 S inversely, Ro=1/G o=2e2/h=12,906 O, with e electron charge and h Plank's constant). In the past conductance histograms and other methods have been used as proof or the signature of conductance quantization.;In the present study a mechanical perturbation approach has been developed as a new method to study conductance quantization in atomic point contacts. The peak height and location in the conductance histogram is shown to vary under different degrees of perturbation (extension or compression of the point contact in the range of 100-500 pm), which clearly dispels the idea of conductance histograms as proof of conductance quantization. The first peak in the conductance histogram is generally believed to be the consequence of single-atom contacts and atomic chains, but here it is shown that a significant portion of the first peak can be attributed to a two-atom configuration. Conductance histograms by themselves only show preferred atomic configurations and cannot even provide information on the stability of those configurations. As a result of these findings, previously used noise analysis methods also do not hold as sufficient proof of quantization of conductance. Noise and strain susceptibility measurements of atomic point contacts show that the atomic structure is the most significant factor in the observed behavior. Measurement of the conductance response to a mechanical perturbation shows that the transmission through a single atom contact saturates at 1 Go. This constitutes the first direct proof that the maximum conductance through one gold atom is 1 Go and that conductance quantization is present in gold atoms.