String tension and stability of magic tip-suspended nanowires

Multishell helical gold nanowires were recently imaged by electron microsco py. We show theoretically that the contact with the gold tips at either end of the wire plays a crucial role and that local minima in the string tensi on rather than the total wire free energy determine the nanowire stability. Density functional electronic structure calculations of the simplest and t hinnest coaxial gold and silver wires of variable radius and chirality were carried out. We found a string tension minimum for a single-tube gold nano wire that is chiral and consists of seven strands, in striking agreement wi th observation. In contrast, no such minimum was found for silver, where th e s-d competition Leading to surface reconstruction is lacking.