No abstract

Using first-principles calculations, we investigate the comparative stability of sub-0.4 nm carbon nanotubes. Compared to the fullerenes C20 and C60, it is found that the most likely carbon nanotubes with a small diameter 0.3 nm are (2,2),(3,1) and (4,0) nanotubes. The spontaneous symmetry breaking of an isolated (2,2) nanotube produces an energy gap at the Fermi level converting it into a semiconductor. The curvature effects lowered the π* band to cross the Fermi energy, leading to the (4,0) nanotube being metallic.

We performed the extensive first-principles simulations for understanding of the stability of complex structures of the nonmolecular nitrogen. We found that the single-bonded cubic phase of nitrogen, so-called cubic gauche structure ͑cg-N͒, is the most stable phase among these phases in high pressure, and predicted that a phase transition of cg-N from nonmetal to metal takes place at the pressure of 600 GPa. The bulk modulus, Young modulus, shear modulus, Poisson ratio, and band gap of cg-N were calculated that are quite well consistent with experimental data.