Transition from tunneling to direct contact in tungsten nanojunctions

Abstract

We apply the mechanically controllable break junctions technique to investigate the transition from tunneling to direct contact in tungsten. This transition is quite different from that of other metals and is determined by the local electronic properties of the tungsten surface and the relief of the electrodes at the point of their closest proximity. When flat surfaces approach each other, an avalanchelike jump to direct contact occurs at anomalously large distances $z\ensuremath{\approx}3--5\mathrm{\AA{}}.$ In contrast, ballistic contact between irregularly shaped electrodes is established without discontinuity in conductance curves, indicating the absence of spontaneous formation of an adhesive neck. Conductance histograms of tungsten are either featureless or show two distinct peaks related to the sequential opening of spatially separated groups of conductance channels. The role of surface states of tungsten and their contribution to the junction conductance at sub-\AA{} electrode separations are discussed.