THE SIGNATURE OF CHEMICAL VALENCE IN THE ELECTRICAL-CONDUCTION THROUGH A SINGLE-ATOM CONTACT

Fabrication of structures at the atomic scale is now possible using st ate-of-the-art techniques for manipulating individual atoms(1), and it may become possible to design electrical circuits atom by atom. A pre requisite for successful design is a knowledge of the relationship bet ween the macroscopic electrical characteristics of such circuits and t he quantum properties of the individual atoms used as building blocks. As a first step, we show here that the chemical valence determines th e conduction properties of the simplest imaginable circuit - a one-ato m contact between two metallic banks. The extended quantum states that carry the current from one bank to the other necessarily proceed thro ugh the valence orbitals of the constriction atom. It thus seems reaso nable to conjecture that the number of current-carrying modes (or 'cha nnels') of a one-atom contact is determined by the number of available valence orbitals, and so should strongly differ for metallic elements in different series of the periodic table. We have tested this conjec ture using scanning tunnelling microscopy and mechanically controllabl e break-junction techniques(2,3) to obtain atomic-size constrictions f or four different metallic elements (Pb, Al, Nb and An), covering a br oad range of valences and orbital structures. Our results demonstrate unambiguously a direct link between valence orbitals and the number of conduction channels in one-atom contacts.