Regularity in the intratube interaction in double-wall carbon nanotubes
  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. 
J. Arvanitidis, D. Christofilos, K. Papagelis, T. Takenobu, Y. Iwasa,   H. Kataura, S. Ves, and G. A. Kourouklis