  The quasi-one-dimensional linear chain compound HfTe3 is experimentally and
theoretically explored in the few- to single-chain limit. Confining the
material within the hollow core of carbon nanotubes allows isolation of the
chains and prevents the rapid oxidation which plagues even bulk HfTe3.
High-resolution transmission electron microscopy combined with density
functional theory calculations reveals that, once the triple-chain limit is
reached, the normally parallel chains spiral about each other, and
simultaneously a short-wavelength trigonal anti-prismatic rocking distortion
occurs that opens a significant energy gap. This results in a size-driven
metal-insulator transition.
