Conduction and mechanical properties of atomic scale gold contacts

Using a dynamic atomic force microscopy (AFM) technique in ultrahigh vacuum, we have directly measured the stiffness and conduction characteristics of a gold tip and sample. The method involves the application of a small sinusoidal oscillating force to the tip at a frequency well below the primary resonance frequency of the cantilever. By measuring the change in amplitude during the approach and retraction of the sample we have a continuous and accurate measure of the contact stiffness. The high sensitivity of this technique has enabled us to measure the mechanical properties of the junction during its initial formation. The most interesting observations are made in the region of initial contact formation where it is not possible to obtain high mechanical sensitivity from the commonly used static force measurement technique. In this region, as the contact is compressed, the contact softens continuously while the conductance remains constant prior to discrete conductance jumps. These are accompanied by simultaneous jumps in stiffness, as predicted by molecular dynamic simulations. Furthermore, the jumps show a strong tendency to half integer values of the conductance quantum. (C) 1999 American Institute of Physics. [S0003-6951(99)01946-4].