Exciton type 2 in graphene bilayer

Excitons play important role in determining the optical properties of condensed mater structures. The exciton type 2 (indirect excitons) is formed when the electron and hole have a difference in phase space. In the frame work of the simplest four band model with two direct or indirect band gaps we investigated and discussed the formation problem of type 2 exciton in graphene bilayers with a finite voltage difference.