Quasiparticle energy and optical excitations of gated bilayer graphene

By employing the first-principles GW-Bethe-Salpeter equation simulation, we obtain the accurate quasiparticle (QP) band gap and optical absorption spectra of gated bilayer graphene. Enhanced electron-electron interactions dramatically enlarge the QP band gap; infrared optical absorption spectra are dictated by bright bound excitons. In particular, the energies of these excited states can be tuned in a substantially wider range, by the gate field, than previous predictions. Our results clearly explain recent experiments and satisfactorily resolve the inconsistency between experimentally measured transport and optical band gaps. Moreover, we predict that the most deeply bound exciton is a dark exciton which is qualitatively different from the hydrogenic model, and its electron and hole are condensed onto opposite graphene layers, respectively. This unique dark exciton not only impacts the exciton dynamics but also provides an exciting opportunity to study entangling exchange effects of many-body physics.