  The lineshapes of the Raman excitation profiles for radial breathing modes in
carbon nanotubes are shown to be strongly affected by interference effects that
arise whenever strong optical transitions are separated by a small energy. This
is the case in metallic zigzag and chiral tubes, where one-dimensional
singularities in the electronic joint density of states are split due to the
trigonal warping of the electronic band structure of two-dimensional graphene.
It is shown that the proper modeling of these interferences is crucial for the
identification of the (n,m) indices using Raman spectroscopy.
