Biexciton State Energies from Many-Body Perturbation Theory Based on Density Functional Theory Simulation

We develop a method for computing self-energy of a biexciton state in a semiconductor nanostructure using many-body perturbation theory (MBPT) based on the density functional theory (DFT) simulation. We compute energies of low-energy biexciton states composed of singlet excitons in the chiral single-wall carbon nanotubes (SWCNT), such as (6,2), (6,5) and (10,5). In all cases we find a small decrease in the biexciton gap: -0.045 $eV$ in (6,2), which is 4.59% of the non-interacting biexciton gap; -0.041 $eV$ in (6,5), which is 4.47% of the non-interacting gap and -0.036 $eV$ in (10,5), which is 4.31%.