From Excitons to topological excitonic states and their fingerprints on the electronic bandstructure

Coulomb-bound electrons and holes forming excitonic states have attracted significant interest given their critical roles in both fundamental science and applications. Whether these excitonic state can be driven in the presence of topological invariants, what properties of the topological state persists and what are their fingerprints in the material’s band structure are all open questions. Here I will discuss some recent work where, by using the ultrafast angle resolved photoemission spectroscopy we study and reveal under which conditions excitonic states can be driven in a topological insulators. The differences with excitonic states in bulk semiconnductors and their fingerprints on the electronic structure are discussed.