  High pressure Raman experiments were performed on bundled double wall carbon
nanotubes (DWCNTs). We observe peculiar regularity in the strength of the
inner-outer tube (intratube) interaction, which is attributed to the interplay
between the tube size and the intratube spacing. Moreover, a pressure-induced
red shift of the band gap energies of the probed inner tubes is deduced from
the Raman spectra. These experimental findings reveal that the interaction
between primary and secondary shells, although generally assumed to be weak,
determines the dynamics in the interior of the DWCNTs.
