CONDITIONS FOR CONDUCTANCE QUANTIZATION IN REALISTIC MODELS OF ATOMIC-SCALE METALLIC CONTACTS

We present a comparative analysis of tight-binding and free-electron c alculations of the conductance of an atomic-scale metallic contact. Th e calculations are based on a full dynamic simulation of the atomic st ructure during the pulloff of the contact, for a range of temperatures . As in previous simulations, we find that the contact evolves through a series of mechanical instabilities and can become highly disordered prior to fracture. Both the mechanical evolution of the contact and t he behavior of the conductance depend strongly on temperature. We find that conductance quantization is destroyed easily by irregularities i n the shape of the contact and, in the tight-binding model, also in th e internal atomic structure of the contact. In the tight-binding calcu lation conductance quantization is seen only at high temperature, when the contact geometry and structure become very regular. With the free -electron model, we see perfectly quantized conductance plateaus just prior to contact fracture, while the plateaus in the earlier history o f the contact are washed out by tunneling. In the free-electron calcul ation, conductance quantization is seen both at low and at high temper ature but is more prominent at high temperature. We use the tight-bind ing and free-electron results for the conductance to obtain a,calibrat ion curve relating the conductance to the constriction width. The calc ulated conductances lie significantly below the Sharvin limit but the inclusion of the first-order semiclassical correction to the Sharvin f ormula greatly improves the agreement.