Imaging the symmetry breaking of molecular orbitals in single-wall carbon nanotubes

Carbon nanotubes have attracted considerable interest for their unique\nelectronic properties. They are fascinating candidates for fundamental studies\nof one dimensional materials as well as for future molecular electronics\napplications. The molecular orbitals of nanotubes are of particular importance\nas they govern the transport properties and the chemical reactivity of the\nsystem. Here we show for the first time a complete experimental investigation\nof molecular orbitals of single wall carbon nanotubes using atomically resolved\nscanning tunneling spectroscopy. Local conductance measurements show\nspectacular carbon-carbon bond asymmetry at the Van Hove singularities for both\nsemiconducting and metallic tubes, demonstrating the symmetry breaking of\nmolecular orbitals in nanotubes. Whatever the tube, only two types of\ncomplementary orbitals are alternatively observed. An analytical tight-binding\nmodel describing the interference patterns of ? orbitals confirmed by ab initio\ncalculations, perfectly reproduces the experimental results.\n