Excitonic dispersion in monolayer C₃N

Monolayer C₃N [1] is a 2D indirect band gap semiconductor with appealing mechanical, thermal, and electronic properties. In this work we present a full characterization of C₃N optical properties, focusing on the so-called momentum-resolved exciton band structure. The study is performed using GW+BSE approach for zero and finite momentum transfer, as implemented in the Yambo code [2].
Excitonic binding energies are indicative of strongly bound excitons, ranging from 0.5 eV to 0.9 eV for the first excitonic states; excitonic wavefunctions are discussed with respect to the symmetry of the crystal.
Excitation energies and amplitudes are computed in order to characterize bright and dark states, and activation of excitonic states is observed for finite transferred momentum: interestingly, we find an indirect excitonic band gap of 0.7 eV, significantly lower than the direct optical band gap (1.8 eV).

[1] J. Mahmood et al., PNAS 113, 7414 (2016).
[2] D Sangalli, et al., J. Physics: Condens. Matter 31 , 325902 (2019).