Fig. 1
Excitonic instability in carbon nanotubes. a Sketch of a suspended armchair carbon nanotube. The field lines of the Coulomb force between electron and hole lie mainly in the vacuum, hence screening is heavily suppressed. b Excitonic instability in the armchair carbon nanotube. The scheme represents the excitation energy ε u of an electron–hole (e–h) pair relative to the noninteracting ground state, a zero-gap semiconductor. In the absence of interaction, the excitation energy ε u of an e–h pair is positive. The long-range interaction may bind e–h pairs close to the Dirac point in momentum space. If an exciton forms, then its excitation energy ε u is negative. This instability leads to the reconstruction of the ground state into an excitonic insulator
