Low Energy Properties of the (n,n) Carbon Nanotubes
  According to band theory, an ideal undoped (n,n) carbon nanotube is metallic. We show that the electron-electron interaction causes it to become Mott insulating with a spin gap. More interestingly, upon doping it develops superconducting fluctuations. 
Yu.A.Krotov, D.-H.Lee, and Steven G.Louie