Chirality dependence of the density-of-states singularities in carbon nanotubes

We derive an approximate analytical expression for the density-of-states singularities in single-walled carbon nanotubes. Our approximation goes beyond the lowest-order, isotropic approach and yields the energy splitting for an arbitary chiral angle in metallic nanotubes. Semiconducting tubes are shown to fall into two classes and have a corresponding energy shift, which should be observable experimentally.