We calculate the band structures of kesterite and stannite Cu 2 ZnSnS 4 and Cu 2 ZnSnSe 4 , using a state-ofthe-art self-consistent GW approach. Our accurate quasiparticle states allow to discuss: the dependence of the gap on the anion displacement; the key-role of the non-locality of the exchange-correlation potential to obtain good structural parameters; the reliability of less expensive hybrid functional and GGA+U approaches. In particular, we show that even if the band gap is correctly reproduced by hybrid functionals, the bandedge corrections are in disagreement with self-consistent GW results, which has decisive implications for the positioning of the defect levels in the band gap.

We have applied the quantum Monte Carlo method and tight-binding modeling to calculate the binding energy of biexcitons in semiconductor carbon nanotubes for a wide range of diameters and chiralities. For typical nanotube diameters we find that biexciton binding energies are much larger than previously predicted from variational methods, which easily brings the biexciton binding energy above the room temperature threshold.

5 pages, 3 figures. Accepted for Phys. Rev. BInternational audienceThe Bethe-Salpeter equation is a widely used approach to describe optical excitations in bulk semiconductors. It leads to spectra that are in very good agreement with experiment, but the price to pay for such accuracy is a very high computational burden. One of the main bottlenecks is the large number of k-points required to obtain converged spectra. In order to circumvent this problem we propose a strategy to solve the Bethe-Salpeter equation based on a double-grid technique coupled to a Wannier interpolation of the Kohn-Sham band structure. This strategy is then benchmarked for a particularly difficult case, the calculation of the absorption spectrum of GaAs, and for the well studied case of Si. The considerable gains observed in these cases fully validate our approach, and open the way for the application of the Bethe-Salpeter equation to large and complex systems