From bulk to monolayer*

In this work, we carry out a detailed theoretical study of the electronic and optical properties of bulk and monolayer bismuth triiodide BiI₃, a layered metal halide, using the ab initio GW+BSE scheme with a full spinorial formulation. We discuss the dimensionality effects in detail along with the role of spin-orbit coupling. Moreover, we compute the exciton dispersion by solving the BSE at finite momentum, also analysing transverse (TE) and longitudinal (LE) excitons, and the longitudinal-transverse splitting at q->0. In bulk, we find a peculiar direction-dependent hybridization of exciton with different character mediated by the long-range Coulomb interaction. Our work provides theoretical support to existing experiment, demonstrate that BiI₃ is an important testbed for the theoretical study of fundamental exciton physics (such as phonon-mediated exciton interaction and localization) and finally confirms that this system is a promising material for the experimental investigation of exciton dynamics (such as tr-ARPES measurements).