Many-Body Calculations of Excitons in Two-Dimensional GaN

We present an ab initio study on quasiparticle (QP) excitations and excitonic effects in two-dimensional (2D) GaN based on density-functional theory and many-body perturbation theory. We calculate the QP band structure using GW approximation, which generates an indirect band gap of 4.83 eV (K & RARR;& UGamma;) for 2D GaN, opening up 1.24 eV with respect to its bulk counterpart. It is shown that the success of plasmon-pole approximation in treating the 2D material benefits considerably from error cancellation. On the other hand, much better gaps, comparable to GW ones, could be obtained by correcting the Kohn-Sham gap with a derivative discontinuity of the exchange-correlation functional at much lower computational cost. To evaluate excitonic effects, we solve the Bethe-Salpeter equation (BSE) starting from Kohn-Sham eigenvalues with a scissors operator to open the single-particle gap. This approach yields an exciton binding energy of 1.23 eV in 2D GaN, which is in good agreement with the highly demanding GW-BSE results. The enhanced excitonic effects due to reduced dimensionality are discussed by comparing the optical spectra from BSE calculations with that by random-phase approximation (RPA) for both the monolayer and bulk GaN in wurtzite phase. Additionally, we find that the spin-orbit splitting of excitonic peaks is noticeable in 2D GaN but buried in the bulk crystal.

Automated summary

In this ab initio study, we investigate quasiparticle (QP) excitations and excitonic effects in two-dimensional GaN using density-functional theory and many-body perturbation theory. By applying the GW approximation, we calculate the QP band structure of 2D GaN and observe an indirect band gap of 4.83 eV, with an increase of 1.24 eV compared to the bulk material. We also demonstrate that a low-cost alternative method, correcting the Kohn-Sham gap with a derivative discontinuity of the exchange-correlation functional, can yield similar results to the GW approach. Furthermore, we evaluate excitonic effects by solving the Bethe-Salpeter equation and find a binding energy of 1.23 eV in 2D GaN, which agrees well with GW-BSE results.