Conductance of single-atom platinum contacts: Voltage dependence of the conductance histogram

The conductance of a single-atom contact is sensitive to the coupling of this contact atom to the atoms in the leads. Notably for the transition metals this gives rise to a considerable spread in the observed conductance values. The mean conductance value and spread can be obtained from the first peak in conductance histograms recorded from a large set of contact-breaking cycles. In contrast to the monovalent metals, this mean value for Pt depends strongly on the applied voltage bias and other experimental conditions and values ranging from about 1 $G_0$ to 2.5 $G_0{(G}_0{=2e}^2/h)$ have been reported. We find that at low bias the first peak in the conductance histogram is centered around 1.5 $G_0.$ However, as the bias increases past 300 mV the peak shifts to 1.8 $G_0.$ Here we show that this bias dependence is due to a geometric effect where monatomic chains are replaced by single-atom contacts, where the former are destabilized by the electron current at high bias.