Conductance histograms, built from thousands of conductance traces recorded during the breaking of a nanowire in a mechanically controllable break junction, are used to investigate the stability of nanowires through statistical analysis. Exceptionally stable wires are formed more often than others and show up as peaks in the histogram. Such experiments on alkali and noble metals have previously revealed a sequence of stable wires, which can be explained by a stabilizing electronic shell effect similar to what is known for clusters. At larger wire diameters a crossover to an atomic shell structure was found, corresponding to the completion of additional atomic layers in crystalline wires.
Our experiments on aluminium nanowires reveal that their stability is also governed by the filling of electronic and atomic shells. The two corresponding series of stable structures are very similar to the ones for noble and alkali metal nanowires. Concerning the electronic shell effect, the nanoscale free-electron model can explain the conductance and geometry (axisymmetric or Jahn–Teller deformed) of stable structures. The atomic shell effect can be understood by considering close-packed hexagonal structures. Moreover, we found a series of stable structures, not reported in any previous experiment, that can be attributed to ‘superdeformed’ nanowires.