Observation of shell structure in sodium nanowires

Abstract

The fact that the rare gas atoms at the end of each row in the periodic table of the elements are exceptionally stable is explained by the closed-shell configuration of their electronic structure. In general, the quantum states of a system of particles in a finite spacial domain form a set of discrete energy eigenvalues, which are usually grouped into bunches of degenerate or close-lying levels, called shells [1]. For fermions this gives rise to a local minimum in the total energy of the system when all states of a given shell are occupied. Shell effects have previously been observed for protons and neutrons in nuclei and for clusters of metal atoms [2], [3], [4]. Here, we report the first observation of a shell effect in an open system, a metal nanowire. When recording the statistical distribution of conductance values observed while pulling off the contact between two bulk sodium metal electrodes, the histogram shows oscillations up to contacts larger than 100 atoms in cross section. The period follows the law expected for the electronic shell-closing effect similar to that determining the “magic numbers” in metal clusters [3], [4]