Observation of Supershell Structure in Alkali Metal Nanowires

Nanowires are formed by indenting and subsequently retracting two pieces of sodium metal. Their cross-section gradually reduces upon retraction and the diameters can be obtained from the conductance. In previous work we have demonstrated that when one constructs a histogram of diameters from large numbers of indentation-retraction cycles, such histograms show a periodic pattern of stable nanowire diameters due to shell structure in the conductance modes. Here, we report the observation of a modulation of this periodic pattern, in agreement with predictions of a supershell structure.