QUANTIZED CONDUCTANCE IN ATOM-SIZED WIRES BETWEEN 2 METALS

We present experimental and theoretical results for the conductance an d mechanical properties of atom-sized wires between two metals. The ex perimental part is based on measurements with a scanning tunneling mic roscope (STM) where a point contact is created by indenting the tip in to a gold surface. When the tip is retracted, a 10-20 Angstrom long na nowire is formed. Our measurements of the conductance of nanowires sho w clear signs of a quantization in units of 2e(2)/h. The scatter aroun d the integer values increases considerably with the number of quanta, and typically it is not possible to observe more than up to four quan ta in these experiments. A detailed discussion is given of the statist ical methods used in the analysis of the experimental data. The theore tical part of the paper addresses some questions posed by the experime nt: Why can conductance quantization be observed, what is the origin o f the scatter in the experimental data, and what is the origin of the scaling of the scattering with the number of conductance quanta? The t heoretical discussion is based on a free-electron-like model where sca ttering from the boundary of the nanowire is included. The configurati ons of the nanowires are deduced from molecular dynamics simulations, which also give information about the mechanical properties of the sys tem. We show that such a model can account semiquantitatively for seve ral of the observed effects. One of the main conclusions of the theore tical analysis is that,; due to the plastic deformation of the nanowir es formed by the STM, the typical length scale of the variations in th e shape of the boundary is not an atomic radius but rather bye times t hat value. This is the reason why scattering is sufficiently small to make conductance quantization observable by STM.