Optical absorption in 4Â Ã (3,3) carbon nanoropes in GW approximation

An ab initio study of the electronic structure and the optical absorption of the various types of nanoropes of the ultrathin (3,3) nanotubes has been performed in the GW approximation. The large curvature effects in the (3,3) nanoropes give rise to a number of features which are opposite to those seen in the large diameter tubes and ropes. The large curvature incurs a decrease in DOS at EF by a factor of two in the asymmetric rope (r.o.=30°) over the symmetric rope, a result which is in contrast to the behaviour seen for comparatively small curvature (10,10) ropes. In the (3,3) ropes which contains large curvature, one observes a strong inter-tube directional covalent binding in contrast to the significant van der Waals interaction present in the large diameter tubes. The non-local energy-dependent self-energy is seen to vary with the wave vector for the various types of the nanoropes characterized by the magnitudes of the relative orientation between the adjacent tubes in a quite similar manner. All the ropes become semiconducting after the inclusion of the many body effects. Strong peaks in absorption along the rope axis appear below 1.0 eV and also in the energy range 2.8-3.0 eV in all the ropes in the generalized gradient approximation (GGA). The GGA peaks lying in the low-energy region are shifted to higher energy side by about 0.1-0.2 eV and those lying in the high-energy region by 0.35 eV on the inclusion of the many-body effects.