Wires of seven atoms- Feynman's very, very small world
The properties of finite many-particle systems do not depend so much on the nature of the particles themselves or the forces acting among them, but on the fact that they are confined and that they are many. In particular, it is well known that the electromagnetic response of finite systems, like the atomic nucleus is strongly influenced by the shape of the system, and by the spill-out of the nucleons from the nuclear surface. In fact, one has observed a conspicuous enhancement of the long-wavelength photo-absorption cross-section in the case of strongly deformed nuclei and of halo nuclei, as compared to the corresponding quantity associated with spherical nuclei lying along the stability valley. Because metals tend to be highly absorbing at long-wavelengths (visible and infrared), the above results clearly suggest that nanometre wires have to be searched among finite atomic systems where electrons feel a strongly deformed mean field, which allows for a conspicuous spill-out of the particles from its surface. Among the systems satisfying these requirements, single-wall nanotubes and linear carbon chains seem to be particularly promising. In fact, we have found from ab initio calculations that they behave as metallic needles when subject to an electromagnetic field. We have furthermore observed that, under standard bias conditions, linear carbon chains are prolific emitters of electrons, the associated currents versus voltage curves displaying a behaviour typical of metallic systems. Single-wall nanotubes and linear carbon chains are thus likely to constitute the ultimate atomic-scale quantum wires.