  A detailed investigation of the Raman response of the inner tube radial
breathing modes (RBMs) in double-wall carbon nanotubes is reported. It revealed
that the number of observed RBMs is two to three times larger than the number
of possible tubes in the studied frequency range. This unexpected increase in
Raman lines is attributed to a splitting of the inner tube response. It is
shown to originate from the possibility that one type of inner tube may form in
different types of outer tubes and the fact that the inner tube RBM frequency
depends on the diameter of the enclosing tube. Finally, a comparison of the
inner tube RBMs and the RBMs of tubes in bundles gave clear evidence that the
interaction in a bundle is stronger than the interaction between inner and
outer tubes.
