Excitons in Glasses from First Principles

Excitons play a central role in the optical and dielectric response of materials, specially for glassy systems, where the localization of electronic states is expected to amplify binding energies and localization of excitons themselves. Probing excitonic behavior computationally is challenging, mainly due to the large supercells needed to represent the glassy state, making the use of high-accuracy, many-body electronic structure methods costly. Here, we represent the glassy system as a collection (composite) of small-cell (few 10s of atoms), periodic local minima on the potential energy surface, obtained via first principles random structure sampling. This approach enables demanding calculations, such as the quasiparticle self-consistent GW and the optical properties at the Bethe-Salpeter level of theory, to be conducted for each local minimum and the properties of the glassy state to be evaluated via ensemble averaging. We show using glassy SiO₂ that the imaginary part of the dielectric function, a sensitive optical observable, computed in this way is in excellent agreement with available experimental data. Our approach also allows for the in-depth analysis of excitons in glassy SiO₂ and the ways the lack of long range order impacts (or not) their behavior. Finally, a viable route for performing accurate theoretical predictions of optical properties including excitonic behavior for any glass is outlined, as well as further strategies to improve the accuracy of results.