Pressure and orientation effects on the electronic structure of carbon nanotube bundles

On the basis of the density functional theory, we study pressure and orientation effects on geometric and electronic structures of crystalline bundles consisting of single-wall carbon nanotubes, (6,0), (8,0), (6,6), and (12,0). We find that mutual orientation angles of adjacent nanotubes in stable geometries correspond to a common intertube atomic arrangement which is similar to art interlayer atomic arrangement of the graphite "AB stacking". In the (6, 6) bundle, it is found that a pseudogap appears at thc Fermi level and its width depends strongly on the intertube orientation. Interestingly, this pesudogap becomes the widest at the most stable intertube orientation which in this case is slightly off the ideal AB-stacking geometry, indicating that the electronic energy gain enhances the pseudogap effect in metallic nanotube bundles. Under lateral pressure. large structural deformation (buckling) is found in the (6 6) bundle and the cross section of the tube in the bundle is deformed from circular to polygonal shapes. In sharp contrast to the, (6, 6) bundle, (n, 0) bundles preserve their circular cross section tinder considerably high pressure (P similar to 30 GPa). Owing to the enhancement of the band dispersion, an insulator-metal transition is found to take place in the (8, 0) bundle even tinder moderate pressure of about 1.5 GPa. In both (6, 6) and (n, 0) cases, polymerization of tubes is observed under the considerably high pressure condition. Unlike C-60 polymers, however, these polymerized nanotube phases are found to be stable only under pressure, Their electronic structures are completely different front that of the isolated nanotubes as in the case of fullerene related materials.