Broken symmetry and pseudogaps in ropes of carbon nanotubes

We investigate the influence of tube-tube interactions in ropes of (10,10) carbon nanotubes, and find that these effects induce a pseudogap in the den sity of states (DOS) of the rope of width 0.1 eV at the Fermi level. In an isolated (n,n) carbon nanotube there are two bands that cross in a linear f ashion at the Fermi level, making the nanotube metallic with a DOS that is constant in a 1.5 eV wide window around the Fermi energy. The presence of t he neighbouring tubes causes these two bands to repel, opening up a band ga p that can be as large as 0.3 eV. The small dispersion in the plane perpend icular to the rope smears out this gap for a rope with a large cross-sectio nal area, and we see a pseudogap at the Fermi energy in the DOS where the D OS falls to one third of its value for the isolated tube. This phenomenon s hould affect many properties of the behavior of ropes of (n,n) nanotubes, w hich should display a more semimetallic character than expected in transpor t and doping experiments, with the existence of both hole and electron carr iers leading to qualitatively different thermopower and Hall-effect behavio rs from those expected for a normal metal. Band repulsion like this can be expected to occur for any tube perturbed by a sufficiently strong interacti on, for example, from contact with a surface or with other tubes.