  Renormalization of quasiparticles and excitons in carbon nanotubes (CNTs)
near a metallic surface has been studied within a many-body formalism using an
embedding approach newly implemented in the GW and Bethe-Salpeter methods. The
quasiparticle bandgap renormalization in semiconducting CNTs is found to scale
as -1/(2h_a), with h_a the apparent nanotube height, and it can exceed half an
eV. Also, the binding energy of excitons is reduced dramatically -by as much as
75%- near the surface. Compensation between quasiparticle and excitonic effects
results in small changes in the optical gap. The important role played by the
nanotube screening response in establishing these effects is emphasized and a
simple electrostatic model with no adjustable parameters explains the results
of state-of-the-art calculations and generalizes them to a large variety of
CNTs.
