Excitons and magnons in 2D crystals with the XATU code

Excitons play a central role in determining the optical properties of two-dimensional (2D) crystals such as hexagonal boron nitride or transition metal dichalcogenides. These properties range from the simplest light absorption to non-linear responses [1]. Methodologically, the Bethe-Salpeter equation (BSE) has become the cornerstone for the computation of excitons. In my talk, I will present the XATU initiative [2], a platform for the implementation of such an equation and the aim to cover the whole range of computational complexity and accuracy, ranging from simple effective models [2] to fully ab initio calculations [3,4] (see Fig. 1). I will emphasise how excitons reshape the linear and non-linear optical responses, particularly the shift current, which plays a central role in anomalous photovoltaics [5]. Finally, I will present our efforts to compute magnons in magnetic 2D crystals from the very same BSE.