Theory of 2D topological excitons
Even in a trivial (non-topological) 2D insulator, the (e,h) binding energy, the exciton-charge and exciton-exciton (molecule-molecule) interactions are far from fully understood. Recently, one of us has developed a Bethe-Salpeter model enabling us to calculate a (density-temperature) phase diagram for excitons, free carriers, bi-molecular exciton-complexes, and (e,h) Cooper pairs in trivial 2D insulators. Here, we will address the peculiarities of bulk, hybrid, and edge-excitons in a topological 2D insulator such as 2D Bi2Se3 (Bi2Te3) nanocrystals, featuring charge, spin and bimolecular dipolar interactions, and how these can be influenced by dielectric proximity by the layer X. The strength (quantified by the absorptance) of topological excitonic bulk and edge transitions will be studied and evaluated with respect to the absorptance quantum pi*(fine structure constant) per exciton volume valid for trivial 2D insulators. The properties of excitons involving the helical edge states are of special interest; the question is if the e-h attraction can result in thermodynamically stable helical excitons in the ground state; as found for more classic systems? In the other case of excited states, can helical excitons be converted into spin-polarized photons?