Excitons in intrinsic and bilayer graphene

Through first-principles calculations with many-body effects included, we have revealed unique excitonic effects in the high-frequency regime ($10~20$ eV) of optical spectra of graphene and bilayer graphene (BLG). Despite their different symmetries, the parallel σ and π* bands result in enhanced excitonic effects in such two-dimensional (2-D) semimetals; one narrowly resonant exciton is discovered to form an isolated peak below the prominent absorption continuum with a surprisingly large binding energy; 270 meV in graphene and 80 meV in BLG. Moreover, because of its extremely weak resonant character, this exciton displays a bound electron-hole wave function and possesses a long intrinsic lifetime, which might be useful in designing optoelectronic applications of graphene.