  The quasiparticle band structure and optical properties of single-walled
zigzag and armchair SiC nanotubes (SiC-NTs) as well as single SiC sheet are
investigated by ab initio many-body calculations using the GW and the GW plus
Bethe-Salpeter equation (GW+BSE) approaches, respectively. Significant GW
quasiparticle corrections of more than 1.0 eV to the Kohn-Sham band gaps from
the local density approximation (LDA) calculations are found. The GW
self-energy corrections transform the SiC sheet from a indirect LDA band gap to
a direct band gap material. Furthermore, the quasiparticle band gaps of SiC-NTs
with different chiralities behave very differently as a function of tube
diameter, and this can be attributed to the difference in the curvature-induced
orbital rehybridization between the different chiral nanotubes. The calculated
optical absorption spectra are dominated by discrete exciton peaks due to
exciton states with large binding energy up to 2.0 eV in the SiC sheet and
SiC-NTs. The formation of strongly bound excitons is attributed to the enhanced
electron-hole interaction in these low dimensional systems. Remarkably, the
excited electron amplitude of the exciton wavefunction is found to peak on the
Si atoms near the hole position (which is on the C site) in the zigzag SiC-NTs,
indicating a charge transfer from an anion (hole) to its neighboring cations by
photoexcitation. In contrast, this pronounced peak structure disappear in the
exciton wavefunction in the armchair SiC-NTs. Furthermore, in the armchair
SiC-NTs, the bound exciton wavefunctions are more localized and also strongly
cylindrically asymmetric.
