Antiferromagnetic excitons in graphene nanodots

Disintegration and formation of excitons in graphene nanodots are investigated within the framework of mean-field approximation to the Hubbard model. Optically active excitons are found to be disintegrated in the non-magnetic nanodots and are shown to form only when the system enters into the antiferromagnetic state. As the Hubbard parameter $U/t$ exceeds a critical value, the nanodot is found to undergo a phase transition from the non-magnetic to an antiferromagnetic phase. Before the phase transition, both optical and quasiparticle gaps are found to be nearly independent of the interaction strength. After the phase transition, however, the quasiparticle gap is revealed to increase more rapidly than the optical gap, which eventually leads to a positive binding energy of the bright exciton in the antiferromagnetic nanodot. The understanding of this extraordinary disintegration and formation of excitons in different magnetic phases of graphene nanodots is believed to be of paramount importance for their potential optoelectronic applications.