A DFT+U study of carbon nanotubes under influence of a gate voltage

A possibility of appearance of antiferromagnetic ordering in armchair carbon nanotubes and graphene is studied using density functional theory within a DFT+U approach. It is shown that the Coulomb repulsion between electrons in the π -orbitals of graphene would need to reach a threshold value of U c = 3 . 6 eV , in order to stabilize the antiferromagnetic order and an energy gap in an electron spectrum at the Fermi level. In the carbon nanotubes the threshold value U c for appearance of the antiferromagnetism decreases with the nanotube’s diameter. For an intermediate value of U < U c the dependence of the energy gap on the diameter of nanotube is well described by a power law in agreement with earlier experimental findings. The antiferromagnetism in the carbon nanotubes is suppressed by departure of the Fermi level from a neutrality point. The latter result can qualitatively explain experimentally observed gate voltage dependence of the energy gap in junctions with the suspended carbon nanotubes as well as the absence of the energy gap in junctions with nanotubes deposited on some substrates.