Novel Excitonic Effects in Graphene and Bilayer Graphene

Through first-principles calculations with many-body effects included, we have revealed unique excitonic effects in the high-frequency regime (10 $\sim $ 20 eV) of optical spectra of graphene and bilayer graphene (BLG). Despite their different symmetries, the parallel $\sigma $ and $\pi $* bands result in enhanced excitonic effects in such two-dimensional semimetals; one narrow 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 exhibits a bound electron-hole wave function.