  Understanding the mechanical properties of nanoscale systems requires a range
of measurement techniques and theoretical approaches to gather the relevant
physical and chemical information. The arrangements of atoms in nanostructures
and macroscopic matter can be different, principally due to the role of surface
energy, but the interplay between atomic and electronic structure in
association with applied mechanical stress can also lead to surprising
differences. For example, metastable structures such as suspended chains of
atoms and helical wires have been produced by the stretching of metal
junctions. Here we report the spontaneous formation of the smallest possible
metal nanotube with a square cross-section during the elongation of silver
nanocontacts. Ab initio calculations and molecular simulations indicate that
the hollow wire forms because this configuration allows the surface energy to
be minimized, and also generates a soft structure capable of absorbing a huge
tensile deformation.
