The study of structure, electronic and optical properties of   double-walled carbon nanotube bundle under hydrostatic pressure
  Combining a classical force field, a tight-binding model, and first-principles calculations, we have studied structural, electronic, and optical properties of double-walled carbon nanotube (DWNT) bundles under hydrostatic pressure. We find that the outer tube acts as a protection shield for the inner tube and the inner tube increases the structure stability and the ability to resist the pressure of the outer tube. Moreover, the collapsed structures of the double-walled carbon nanotube bundle called ``parallel'' and ``in-between'' are more stable than the one called ``herringbone''. The structural phase transition induces a pseudogap along symmetry line \textit{$\Gamma $X}. Furthermore, the optical properties change greatly after the collapse and a strong anisotropy appears in the collapsed structure. This provides an efficient experimental way to detect structural phase transitions in DWNT bundles. 
Xiaoping Yang, Gang Wu