Connective neck evolution and conductance steps in hot point contacts

Dynamic evolution of the connective neck in Al and Pb mechanically controllable break junctions was studied during continuous approach of electrodes at bias voltages ${V}_{b}$ up to a few hundred millivolt. A high level of power dissipation ${(10}^{\ensuremath{-}4}\char21{}{10}^{\ensuremath{-}3}\mathrm{W})$ and high current density $(j\ensuremath{\gtrsim}{10}^{10}{\mathrm{A}/\mathrm{c}\mathrm{m}}^{2})$ in the constriction lead to overheating of the contact area, electromigration, and current-enhanced diffusion of atoms out of the ``hot spot.'' At a low electrode approach rate (\ensuremath{\sim}10\char21{}50 pm/s) the transverse dimension of the neck and the conductance of the junction depend on ${V}_{b}$ and remain nearly constant over the approach distance of 10\char21{}30 nm. For ${V}_{b}g300\mathrm{mV}$ the connective neck consists of a few atoms only and the quantum nature of conductance manifests itself in abrupt steps and reversible jumps between two or more levels. These features are related to an ever changing number of individual conductance channels due to the continuous rearrangement in atomic configuration of the neck, the recurring motion of atoms between metastable states, the formation and breaking of isolated one-atom contacts, and the switching between energetically preferable neck geometries.