  We have used resonant Raman scattering spectroscopy to fully analyze the
relative abundances of different (n,m) species in single-walled carbon nanotube
samples that are metallically enriched by density gradient ultracentrifugation.
Strikingly, the data clearly show that our density gradient ultracentrifugation
process enriches the metallic fractions in armchair and near-armchair species.
We observe that armchair carbon nanotubes constitute more than 50% of each (2n
+ m) family.
